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Sudoku | Backtracking-7

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  • Difficulty Level : Hard
  • Last Updated : 01 Nov, 2022
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Given a partially filled 9×9 2D array ‘grid[9][9]’, the goal is to assign digits (from 1 to 9) to the empty cells so that every row, column, and subgrid of size 3×3 contains exactly one instance of the digits from 1 to 9. 

Examples: 

Input: grid
{ {3, 0, 6, 5, 0, 8, 4, 0, 0},
{5, 2, 0, 0, 0, 0, 0, 0, 0},
{0, 8, 7, 0, 0, 0, 0, 3, 1},
{0, 0, 3, 0, 1, 0, 0, 8, 0},
{9, 0, 0, 8, 6, 3, 0, 0, 5},
{0, 5, 0, 0, 9, 0, 6, 0, 0}, 
{1, 3, 0, 0, 0, 0, 2, 5, 0},
{0, 0, 0, 0, 0, 0, 0, 7, 4},
{0, 0, 5, 2, 0, 6, 3, 0, 0} }
Output:
3 1 6 5 7 8 4 9 2
5 2 9 1 3 4 7 6 8
4 8 7 6 2 9 5 3 1
2 6 3 4 1 5 9 8 7
9 7 4 8 6 3 1 2 5
8 5 1 7 9 2 6 4 3
1 3 8 9 4 7 2 5 6
6 9 2 3 5 1 8 7 4
7 4 5 2 8 6 3 1 9
Explanation: Each row, column and 3*3 box of the output matrix contains unique numbers.

Input: grid
{ { 3, 1, 6, 5, 7, 8, 4, 9, 2 },
{ 5, 2, 9, 1, 3, 4, 7, 6, 8 },
{ 4, 8, 7, 6, 2, 9, 5, 3, 1 },
{ 2, 6, 3, 0, 1, 5, 9, 8, 7 },
{ 9, 7, 4, 8, 6, 0, 1, 2, 5 },
{ 8, 5, 1, 7, 9, 2, 6, 4, 3 },
{ 1, 3, 8, 0, 4, 7, 2, 0, 6 },
{ 6, 9, 2, 3, 5, 1, 8, 7, 4 },
{ 7, 4, 5, 0, 8, 6, 3, 1, 0 } };
Output:
3 1 6 5 7 8 4 9 2 
5 2 9 1 3 4 7 6 8 
4 8 7 6 2 9 5 3 1 
2 6 3 4 1 5 9 8 7
9 7 4 8 6 3 1 2 5
8 5 1 7 9 2 6 4 3
1 3 8 9 4 7 2 5 6
6 9 2 3 5 1 8 7 4
7 4 5 2 8 6 3 1 9 
Explanation: Each row, column and 3*3 box of the output matrix contains unique numbers.

Recommended Practice

Naive Approach:

The naive approach is to generate all possible configurations of numbers from 1 to 9 to fill the empty cells. Try every configuration one by one until the correct configuration is found, i.e. for every unassigned position fill the position with a number from 1 to 9. After filling all the unassigned positions check if the matrix is safe or not. If safe print else recurs for other cases.

Follow the steps below to solve the problem:

  • Create a function that checks if the given matrix is valid sudoku or not. Keep Hashmap for the row, column and boxes. If any number has a frequency greater than 1 in the hashMap return false else return true;
  • Create a recursive function that takes a grid and the current row and column index.
  • Check some base cases. 
    • If the index is at the end of the matrix, i.e. i=N-1 and j=N then check if the grid is safe or not, if safe print the grid and return true else return false. 
    • The other base case is when the value of column is N, i.e j = N, then move to next row, i.e. i++ and j = 0.
  • If the current index is not assigned then fill the element from 1 to 9 and recur for all 9 cases with the index of next element, i.e. i, j+1. if the recursive call returns true then break the loop and return true.
  • If the current index is assigned then call the recursive function with the index of the next element, i.e. i, j+1

Below is the implementation of the above approach:

C++




#include <iostream>
 
using namespace std;
 
// N is the size of the 2D matrix   N*N
#define N 9
 
/* A utility function to print grid */
void print(int arr[N][N])
{
    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
            cout << arr[i][j] << " ";
        cout << endl;
    }
}
 
// Checks whether it will be
// legal to assign num to the
// given row, col
bool isSafe(int grid[N][N], int row,
                       int col, int num)
{
     
    // Check if we find the same num
    // in the similar row , we
    // return false
    for (int x = 0; x <= 8; x++)
        if (grid[row][x] == num)
            return false;
 
    // Check if we find the same num in
    // the similar column , we
    // return false
    for (int x = 0; x <= 8; x++)
        if (grid[x][col] == num)
            return false;
 
    // Check if we find the same num in
    // the particular 3*3 matrix,
    // we return false
    int startRow = row - row % 3,
            startCol = col - col % 3;
   
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            if (grid[i + startRow][j +
                            startCol] == num)
                return false;
 
    return true;
}
 
/* Takes a partially filled-in grid and attempts
to assign values to all unassigned locations in
such a way to meet the requirements for
Sudoku solution (non-duplication across rows,
columns, and boxes) */
bool solveSudoku(int grid[N][N], int row, int col)
{
    // Check if we have reached the 8th
    // row and 9th column (0
    // indexed matrix) , we are
    // returning true to avoid
    // further backtracking
    if (row == N - 1 && col == N)
        return true;
 
    // Check if column value  becomes 9 ,
    // we move to next row and
    //  column start from 0
    if (col == N) {
        row++;
        col = 0;
    }
   
    // Check if the current position of
    // the grid already contains
    // value >0, we iterate for next column
    if (grid[row][col] > 0)
        return solveSudoku(grid, row, col + 1);
 
    for (int num = 1; num <= N; num++)
    {
         
        // Check if it is safe to place
        // the num (1-9)  in the
        // given row ,col  ->we
        // move to next column
        if (isSafe(grid, row, col, num))
        {
             
           /* Assigning the num in
              the current (row,col)
              position of the grid
              and assuming our assigned
              num in the position
              is correct     */
            grid[row][col] = num;
           
            //  Checking for next possibility with next
            //  column
            if (solveSudoku(grid, row, col + 1))
                return true;
        }
       
        // Removing the assigned num ,
        // since our assumption
        // was wrong , and we go for
        // next assumption with
        // diff num value
        grid[row][col] = 0;
    }
    return false;
}
 
// Driver Code
int main()
{
    // 0 means unassigned cells
    int grid[N][N] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                       { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                       { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                       { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                       { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                       { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                       { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                       { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                       { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
    if (solveSudoku(grid, 0, 0))
        print(grid);
    else
        cout << "no solution  exists " << endl;
 
    return 0;
    // This is code is contributed by Pradeep Mondal P
}


C




#include <stdio.h>
#include <stdlib.h>
 
// N is the size of the 2D matrix   N*N
#define N 9
 
/* A utility function to print grid */
void print(int arr[N][N])
{
     for (int i = 0; i < N; i++)
      {
         for (int j = 0; j < N; j++)
            printf("%d ",arr[i][j]);
         printf("\n");
       }
}
 
// Checks whether it will be legal 
// to assign num to the
// given row, col
int isSafe(int grid[N][N], int row,
                       int col, int num)
{
     
    // Check if we find the same num
    // in the similar row , we return 0
    for (int x = 0; x <= 8; x++)
        if (grid[row][x] == num)
            return 0;
 
    // Check if we find the same num in the
    // similar column , we return 0
    for (int x = 0; x <= 8; x++)
        if (grid[x][col] == num)
            return 0;
 
    // Check if we find the same num in the
    // particular 3*3 matrix, we return 0
    int startRow = row - row % 3,
                 startCol = col - col % 3;
   
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            if (grid[i + startRow][j +
                          startCol] == num)
                return 0;
 
    return 1;
}
 
/* Takes a partially filled-in grid and attempts
to assign values to all unassigned locations in
such a way to meet the requirements for
Sudoku solution (non-duplication across rows,
columns, and boxes) */
int solveSudoku(int grid[N][N], int row, int col)
{
     
    // Check if we have reached the 8th row
    // and 9th column (0
    // indexed matrix) , we are
    // returning true to avoid
    // further backtracking
    if (row == N - 1 && col == N)
        return 1;
 
    //  Check if column value  becomes 9 ,
    //  we move to next row and
    //  column start from 0
    if (col == N)
    {
        row++;
        col = 0;
    }
   
    // Check if the current position
    // of the grid already contains
    // value >0, we iterate for next column
    if (grid[row][col] > 0)
        return solveSudoku(grid, row, col + 1);
 
    for (int num = 1; num <= N; num++)
    {
         
        // Check if it is safe to place
        // the num (1-9)  in the
        // given row ,col  ->we move to next column
        if (isSafe(grid, row, col, num)==1)
        {
            /* assigning the num in the
               current (row,col)
               position of the grid
               and assuming our assigned num
               in the position
               is correct     */
            grid[row][col] = num;
           
            //  Checking for next possibility with next
            //  column
            if (solveSudoku(grid, row, col + 1)==1)
                return 1;
        }
       
        // Removing the assigned num ,
        // since our assumption
        // was wrong , and we go for next
        // assumption with
        // diff num value
        grid[row][col] = 0;
    }
    return 0;
}
 
int main()
{
    // 0 means unassigned cells
    int grid[N][N] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                       { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                       { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                       { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                       { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                       { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                       { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                       { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                       { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
    if (solveSudoku(grid, 0, 0)==1)
        print(grid);
    else
        printf("No solution exists");
 
    return 0;
    // This is code is contributed by Pradeep Mondal P
}


Java




// Java program for above approach
public class Sudoku {
 
    // N is the size of the 2D matrix   N*N
    static int N = 9;
 
    /* Takes a partially filled-in grid and attempts
    to assign values to all unassigned locations in
    such a way to meet the requirements for
    Sudoku solution (non-duplication across rows,
    columns, and boxes) */
    static boolean solveSudoku(int grid[][], int row,
                               int col)
    {
 
        /*if we have reached the 8th
           row and 9th column (0
           indexed matrix) ,
           we are returning true to avoid further
           backtracking       */
        if (row == N - 1 && col == N)
            return true;
 
        // Check if column value  becomes 9 ,
        // we move to next row
        // and column start from 0
        if (col == N) {
            row++;
            col = 0;
        }
 
        // Check if the current position
        // of the grid already
        // contains value >0, we iterate
        // for next column
        if (grid[row][col] != 0)
            return solveSudoku(grid, row, col + 1);
 
        for (int num = 1; num < 10; num++) {
 
            // Check if it is safe to place
            // the num (1-9)  in the
            // given row ,col ->we move to next column
            if (isSafe(grid, row, col, num)) {
 
                /*  assigning the num in the current
                (row,col)  position of the grid and
                assuming our assigned num in the position
                is correct */
                grid[row][col] = num;
 
                // Checking for next
                // possibility with next column
                if (solveSudoku(grid, row, col + 1))
                    return true;
            }
            /* removing the assigned num , since our
               assumption was wrong , and we go for next
               assumption with diff num value   */
            grid[row][col] = 0;
        }
        return false;
    }
 
    /* A utility function to print grid */
    static void print(int[][] grid)
    {
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++)
                System.out.print(grid[i][j] + " ");
            System.out.println();
        }
    }
 
    // Check whether it will be legal
    // to assign num to the
    // given row, col
    static boolean isSafe(int[][] grid, int row, int col,
                          int num)
    {
 
        // Check if we find the same num
        // in the similar row , we
        // return false
        for (int x = 0; x <= 8; x++)
            if (grid[row][x] == num)
                return false;
 
        // Check if we find the same num
        // in the similar column ,
        // we return false
        for (int x = 0; x <= 8; x++)
            if (grid[x][col] == num)
                return false;
 
        // Check if we find the same num
        // in the particular 3*3
        // matrix, we return false
        int startRow = row - row % 3, startCol
                                      = col - col % 3;
        for (int i = 0; i < 3; i++)
            for (int j = 0; j < 3; j++)
                if (grid[i + startRow][j + startCol] == num)
                    return false;
 
        return true;
    }
  
    // Driver Code
    public static void main(String[] args)
    {
        int grid[][] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                         { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                         { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                         { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                         { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                         { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                         { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                         { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                         { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
        if (solveSudoku(grid, 0, 0))
            print(grid);
        else
            System.out.println("No Solution exists");
    }
    // This is code is contributed by Pradeep Mondal P
}


Python3




# N is the size of the 2D matrix   N*N
N = 9
 
# A utility function to print grid
def printing(arr):
    for i in range(N):
        for j in range(N):
            print(arr[i][j], end = " ")
        print()
 
# Checks whether it will be
# legal to assign num to the
# given row, col
def isSafe(grid, row, col, num):
   
    # Check if we find the same num
    # in the similar row , we
    # return false
    for x in range(9):
        if grid[row][x] == num:
            return False
 
    # Check if we find the same num in
    # the similar column , we
    # return false
    for x in range(9):
        if grid[x][col] == num:
            return False
 
    # Check if we find the same num in
    # the particular 3*3 matrix,
    # we return false
    startRow = row - row % 3
    startCol = col - col % 3
    for i in range(3):
        for j in range(3):
            if grid[i + startRow][j + startCol] == num:
                return False
    return True
 
# Takes a partially filled-in grid and attempts
# to assign values to all unassigned locations in
# such a way to meet the requirements for
# Sudoku solution (non-duplication across rows,
# columns, and boxes) */
def solveSudoku(grid, row, col):
   
    # Check if we have reached the 8th
    # row and 9th column (0
    # indexed matrix) , we are
    # returning true to avoid
    # further backtracking
    if (row == N - 1 and col == N):
        return True
       
    # Check if column value  becomes 9 ,
    # we move to next row and
    # column start from 0
    if col == N:
        row += 1
        col = 0
 
    # Check if the current position of
    # the grid already contains
    # value >0, we iterate for next column
    if grid[row][col] > 0:
        return solveSudoku(grid, row, col + 1)
    for num in range(1, N + 1, 1):
       
        # Check if it is safe to place
        # the num (1-9)  in the
        # given row ,col  ->we
        # move to next column
        if isSafe(grid, row, col, num):
           
            # Assigning the num in
            # the current (row,col)
            # position of the grid
            # and assuming our assigned
            # num in the position
            # is correct
            grid[row][col] = num
 
            # Checking for next possibility with next
            # column
            if solveSudoku(grid, row, col + 1):
                return True
 
        # Removing the assigned num ,
        # since our assumption
        # was wrong , and we go for
        # next assumption with
        # diff num value
        grid[row][col] = 0
    return False
 
# Driver Code
 
# 0 means unassigned cells
grid = [[3, 0, 6, 5, 0, 8, 4, 0, 0],
        [5, 2, 0, 0, 0, 0, 0, 0, 0],
        [0, 8, 7, 0, 0, 0, 0, 3, 1],
        [0, 0, 3, 0, 1, 0, 0, 8, 0],
        [9, 0, 0, 8, 6, 3, 0, 0, 5],
        [0, 5, 0, 0, 9, 0, 6, 0, 0],
        [1, 3, 0, 0, 0, 0, 2, 5, 0],
        [0, 0, 0, 0, 0, 0, 0, 7, 4],
        [0, 0, 5, 2, 0, 6, 3, 0, 0]]
 
if (solveSudoku(grid, 0, 0)):
    printing(grid)
else:
    print("no solution  exists ")
 
    # This code is contributed by sudhanshgupta2019a


C#




// C# program for above approach
using System;
class GFG {
 
  // N is the size of the 2D matrix   N*N
  static int N = 9;
 
  /* Takes a partially filled-in grid and attempts
    to assign values to all unassigned locations in
    such a way to meet the requirements for
    Sudoku solution (non-duplication across rows,
    columns, and boxes) */
  static bool solveSudoku(int[,] grid, int row,
                          int col)
  {
 
    /*if we have reached the 8th
           row and 9th column (0
           indexed matrix) ,
           we are returning true to avoid further
           backtracking       */
    if (row == N - 1 && col == N)
      return true;
 
    // Check if column value  becomes 9 ,
    // we move to next row
    // and column start from 0
    if (col == N) {
      row++;
      col = 0;
    }
 
    // Check if the current position
    // of the grid already
    // contains value >0, we iterate
    // for next column
    if (grid[row,col] != 0)
      return solveSudoku(grid, row, col + 1);
 
    for (int num = 1; num < 10; num++) {
 
      // Check if it is safe to place
      // the num (1-9)  in the
      // given row ,col ->we move to next column
      if (isSafe(grid, row, col, num)) {
 
        /*  assigning the num in the current
                (row,col)  position of the grid and
                assuming our assigned num in the position
                is correct */
        grid[row,col] = num;
 
        // Checking for next
        // possibility with next column
        if (solveSudoku(grid, row, col + 1))
          return true;
      }
      /* removing the assigned num , since our
               assumption was wrong , and we go for next
               assumption with diff num value   */
      grid[row,col] = 0;
    }
    return false;
  }
 
  /* A utility function to print grid */
  static void print(int[,] grid)
  {
    for (int i = 0; i < N; i++) {
      for (int j = 0; j < N; j++)
        Console.Write(grid[i,j] + " ");
      Console.WriteLine();
    }
  }
 
  // Check whether it will be legal
  // to assign num to the
  // given row, col
  static bool isSafe(int[,] grid, int row, int col,
                     int num)
  {
 
    // Check if we find the same num
    // in the similar row , we
    // return false
    for (int x = 0; x <= 8; x++)
      if (grid[row,x] == num)
        return false;
 
    // Check if we find the same num
    // in the similar column ,
    // we return false
    for (int x = 0; x <= 8; x++)
      if (grid[x,col] == num)
        return false;
 
    // Check if we find the same num
    // in the particular 3*3
    // matrix, we return false
    int startRow = row - row % 3, startCol
      = col - col % 3;
    for (int i = 0; i < 3; i++)
      for (int j = 0; j < 3; j++)
        if (grid[i + startRow,j + startCol] == num)
          return false;
 
    return true;
  }
 
  // Driver code
  static void Main() {
    int[,] grid = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                   { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                   { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                   { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                   { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                   { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                   { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                   { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                   { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
    if (solveSudoku(grid, 0, 0))
      print(grid);
    else
      Console.WriteLine("No Solution exists");
  }
}
 
// This code is contributed by divyesh072019.


Javascript




<script>
 
// Javascript program for above approach
 
// N is the size of the 2D matrix   N*N
let N = 9;
 
/* Takes a partially filled-in grid and attempts
    to assign values to all unassigned locations in
    such a way to meet the requirements for
    Sudoku solution (non-duplication across rows,
    columns, and boxes) */
function solveSudoku(grid, row, col)
{
     
    /* If we have reached the 8th
       row and 9th column (0
       indexed matrix) ,
       we are returning true to avoid further
       backtracking       */
    if (row == N - 1 && col == N)
        return true;
 
    // Check if column value  becomes 9 ,
    // we move to next row
    // and column start from 0
    if (col == N)
    {
        row++;
        col = 0;
    }
 
    // Check if the current position
    // of the grid already
    // contains value >0, we iterate
    // for next column
    if (grid[row][col] != 0)
        return solveSudoku(grid, row, col + 1);
 
    for(let num = 1; num < 10; num++)
    {
         
        // Check if it is safe to place
        // the num (1-9)  in the given
        // row ,col ->we move to next column
        if (isSafe(grid, row, col, num))
        {
             
            /*  assigning the num in the current
            (row,col)  position of the grid and
            assuming our assigned num in the position
            is correct */
            grid[row][col] = num;
 
            // Checking for next
            // possibility with next column
            if (solveSudoku(grid, row, col + 1))
                return true;
        }
         
        /* removing the assigned num , since our
           assumption was wrong , and we go for next
           assumption with diff num value   */
        grid[row][col] = 0;
    }
    return false;
}
 
/* A utility function to print grid */
function print(grid)
{
    for(let i = 0; i < N; i++)
    {
        for(let j = 0; j < N; j++)
            document.write(grid[i][j] + " ");
             
        document.write("<br>");
    }
}
 
// Check whether it will be legal
// to assign num to the
// given row, col
function isSafe(grid, row, col, num)
{
     
    // Check if we find the same num
    // in the similar row , we
    // return false
    for(let x = 0; x <= 8; x++)
        if (grid[row][x] == num)
            return false;
 
    // Check if we find the same num
    // in the similar column ,
    // we return false
    for(let x = 0; x <= 8; x++)
        if (grid[x][col] == num)
            return false;
 
    // Check if we find the same num
    // in the particular 3*3
    // matrix, we return false
    let startRow = row - row % 3,
        startCol = col - col % 3;
         
    for(let i = 0; i < 3; i++)
        for(let j = 0; j < 3; j++)
            if (grid[i + startRow][j + startCol] == num)
                return false;
 
    return true;
}
 
// Driver Code
let grid = [ [ 3, 0, 6, 5, 0, 8, 4, 0, 0 ],
             [ 5, 2, 0, 0, 0, 0, 0, 0, 0 ],
             [ 0, 8, 7, 0, 0, 0, 0, 3, 1 ],
             [ 0, 0, 3, 0, 1, 0, 0, 8, 0 ],
             [ 9, 0, 0, 8, 6, 3, 0, 0, 5 ],
             [ 0, 5, 0, 0, 9, 0, 6, 0, 0 ],
             [ 1, 3, 0, 0, 0, 0, 2, 5, 0 ],
             [ 0, 0, 0, 0, 0, 0, 0, 7, 4 ],
             [ 0, 0, 5, 2, 0, 6, 3, 0, 0 ] ]
  
if (solveSudoku(grid, 0, 0))
    print(grid)
else
    document.write("no solution  exists ")
 
// This code is contributed by rag2127
 
</script>


Output

3 1 6 5 7 8 4 9 2 
5 2 9 1 3 4 7 6 8 
4 8 7 6 2 9 5 3 1 
2 6 3 4 1 5 9 8 7 
9 7 4 8 6 3 1 2 5 
8 5 1 7 9 2 6 4 3 
1 3 8 9 4 7 2 5 6 
6 9 2 3 5 1 8 7 4 
7 4 5 2 8 6 3 1 9

Time complexity: O(9(N*N)), For every unassigned index, there are 9 possible options so the time complexity is O(9^(n*n)).
Space Complexity: O(N*N), To store the output array a matrix is needed.

Sudoku using Backtracking:

Like all other Backtracking problems, Sudoku can be solved by assigning numbers one by one to empty cells. Before assigning a number, check whether it is safe to assign. 

Check that the same number is not present in the current row, current column and current 3X3 subgrid. After checking for safety, assign the number, and recursively check whether this assignment leads to a solution or not. If the assignment doesn’t lead to a solution, then try the next number for the current empty cell. And if none of the number (1 to 9) leads to a solution, return false and print no solution exists.

Follow the steps below to solve the problem:

  • Create a function that checks after assigning the current index the grid becomes unsafe or not. Keep Hashmap for a row, column and boxes. If any number has a frequency greater than 1 in the hashMap return false else return true; hashMap can be avoided by using loops.
  • Create a recursive function that takes a grid.
  • Check for any unassigned location. 
    • If present then assigns a number from 1 to 9.
    • Check if assigning the number to current index makes the grid unsafe or not. 
    • If safe then recursively call the function for all safe cases from 0 to 9.
    • If any recursive call returns true, end the loop and return true. If no recursive call returns true then return false.
  • If there is no unassigned location then return true.

Below is the implementation of the above approach:

C++




// A Backtracking program in
// C++ to solve Sudoku problem
#include <bits/stdc++.h>
using namespace std;
 
// UNASSIGNED is used for empty
// cells in sudoku grid
#define UNASSIGNED 0
 
// N is used for the size of Sudoku grid.
// Size will be NxN
#define N 9
 
// This function finds an entry in grid
// that is still unassigned
bool FindUnassignedLocation(int grid[N][N],
                            int& row, int& col);
 
// Checks whether it will be legal
// to assign num to the given row, col
bool isSafe(int grid[N][N], int row,
            int col, int num);
 
/* Takes a partially filled-in grid and attempts
to assign values to all unassigned locations in
such a way to meet the requirements for
Sudoku solution (non-duplication across rows,
columns, and boxes) */
bool SolveSudoku(int grid[N][N])
{
    int row, col;
 
    // If there is no unassigned location,
    // we are done
    if (!FindUnassignedLocation(grid, row, col))
        // success!
        return true;
 
    // Consider digits 1 to 9
    for (int num = 1; num <= 9; num++)
    {
         
        // Check if looks promising
        if (isSafe(grid, row, col, num))
        {
             
            // Make tentative assignment
            grid[row][col] = num;
 
            // Return, if success
            if (SolveSudoku(grid))
                return true;
 
            // Failure, unmake & try again
            grid[row][col] = UNASSIGNED;
        }
    }
    
    // This triggers backtracking
    return false;
}
 
/* Searches the grid to find an entry that is
still unassigned. If found, the reference
parameters row, col will be set the location
that is unassigned, and true is returned.
If no unassigned entries remain, false is returned. */
bool FindUnassignedLocation(int grid[N][N],
                            int& row, int& col)
{
    for (row = 0; row < N; row++)
        for (col = 0; col < N; col++)
            if (grid[row][col] == UNASSIGNED)
                return true;
    return false;
}
 
/* Returns a boolean which indicates whether
an assigned entry in the specified row matches
the given number. */
bool UsedInRow(int grid[N][N], int row, int num)
{
    for (int col = 0; col < N; col++)
        if (grid[row][col] == num)
            return true;
    return false;
}
 
/* Returns a boolean which indicates whether
an assigned entry in the specified column
matches the given number. */
bool UsedInCol(int grid[N][N], int col, int num)
{
    for (int row = 0; row < N; row++)
        if (grid[row][col] == num)
            return true;
    return false;
}
 
/* Returns a boolean which indicates whether
an assigned entry within the specified 3x3 box
matches the given number. */
bool UsedInBox(int grid[N][N], int boxStartRow,
               int boxStartCol, int num)
{
    for (int row = 0; row < 3; row++)
        for (int col = 0; col < 3; col++)
            if (grid[row + boxStartRow]
                    [col + boxStartCol] ==
                                       num)
                return true;
    return false;
}
 
/* Returns a boolean which indicates whether
it will be legal to assign num to the given
row, col location. */
bool isSafe(int grid[N][N], int row,
            int col, int num)
{
    /* Check if 'num' is not already placed in
    current row, current column
    and current 3x3 box */
    return !UsedInRow(grid, row, num)
           && !UsedInCol(grid, col, num)
           && !UsedInBox(grid, row - row % 3,
                         col - col % 3, num)
           && grid[row][col] == UNASSIGNED;
}
 
/* A utility function to print grid */
void printGrid(int grid[N][N])
{
    for (int row = 0; row < N; row++)
    {
        for (int col = 0; col < N; col++)
            cout << grid[row][col] << " ";
        cout << endl;
    }
}
 
// Driver Code
int main()
{
    // 0 means unassigned cells
    int grid[N][N] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                       { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                       { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                       { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                       { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                       { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                       { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                       { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                       { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
    if (SolveSudoku(grid) == true)
        printGrid(grid);
    else
        cout << "No solution exists";
 
    return 0;
}
 
// This is code is contributed by rathbhupendra


Java




/* A Backtracking program in
Java to solve Sudoku problem */
class GFG
{
    public static boolean isSafe(int[][] board,
                                 int row, int col,
                                 int num)
    {
        // Row has the unique (row-clash)
        for (int d = 0; d < board.length; d++)
        {
             
            // Check if the number we are trying to
            // place is already present in
            // that row, return false;
            if (board[row][d] == num) {
                return false;
            }
        }
 
        // Column has the unique numbers (column-clash)
        for (int r = 0; r < board.length; r++)
        {
             
            // Check if the number
            // we are trying to
            // place is already present in
            // that column, return false;
            if (board[r][col] == num)
            {
                return false;
            }
        }
 
        // Corresponding square has
        // unique number (box-clash)
        int sqrt = (int)Math.sqrt(board.length);
        int boxRowStart = row - row % sqrt;
        int boxColStart = col - col % sqrt;
 
        for (int r = boxRowStart;
             r < boxRowStart + sqrt; r++)
        {
            for (int d = boxColStart;
                 d < boxColStart + sqrt; d++)
            {
                if (board[r][d] == num)
                {
                    return false;
                }
            }
        }
 
        // if there is no clash, it's safe
        return true;
    }
 
    public static boolean solveSudoku(
        int[][] board, int n)
    {
        int row = -1;
        int col = -1;
        boolean isEmpty = true;
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (board[i][j] == 0)
                {
                    row = i;
                    col = j;
 
                    // We still have some remaining
                    // missing values in Sudoku
                    isEmpty = false;
                    break;
                }
            }
            if (!isEmpty) {
                break;
            }
        }
 
        // No empty space left
        if (isEmpty)
        {
            return true;
        }
 
        // Else for each-row backtrack
        for (int num = 1; num <= n; num++)
        {
            if (isSafe(board, row, col, num))
            {
                board[row][col] = num;
                if (solveSudoku(board, n))
                {
                    // print(board, n);
                    return true;
                }
                else
                {
                    // replace it
                    board[row][col] = 0;
                }
            }
        }
        return false;
    }
 
    public static void print(
        int[][] board, int N)
    {
         
        // We got the answer, just print it
        for (int r = 0; r < N; r++)
        {
            for (int d = 0; d < N; d++)
            {
                System.out.print(board[r][d]);
                System.out.print(" ");
            }
            System.out.print("\n");
 
            if ((r + 1) % (int)Math.sqrt(N) == 0)
            {
                System.out.print("");
            }
        }
    }
 
    // Driver Code
    public static void main(String args[])
    {
 
        int[][] board = new int[][] {
            { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
            { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
            { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
            { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
            { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
            { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
            { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
            { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
            { 0, 0, 5, 2, 0, 6, 3, 0, 0 }
        };
        int N = board.length;
 
        if (solveSudoku(board, N))
        {
            // print solution
            print(board, N);
        }
        else {
            System.out.println("No solution");
        }
    }
}
 
// This code is contributed
// by MohanDas


Python3




# A Backtracking program
# in Python to solve Sudoku problem
 
# A Utility Function to print the Grid
def print_grid(arr):
    for i in range(9):
        for j in range(9):
            print (arr[i][j], end = " "),
        print ()
 
         
# Function to Find the entry in
# the Grid that is still  not used
# Searches the grid to find an
# entry that is still unassigned. If
# found, the reference parameters
# row, col will be set the location
# that is unassigned, and true is
# returned. If no unassigned entries
# remains, false is returned.
# 'l' is a list  variable that has
# been passed from the solve_sudoku function
# to keep track of incrementation
# of Rows and Columns
def find_empty_location(arr, l):
    for row in range(9):
        for col in range(9):
            if(arr[row][col]== 0):
                l[0]= row
                l[1]= col
                return True
    return False
 
# Returns a boolean which indicates
# whether any assigned entry
# in the specified row matches
# the given number.
def used_in_row(arr, row, num):
    for i in range(9):
        if(arr[row][i] == num):
            return True
    return False
 
# Returns a boolean which indicates
# whether any assigned entry
# in the specified column matches
# the given number.
def used_in_col(arr, col, num):
    for i in range(9):
        if(arr[i][col] == num):
            return True
    return False
 
# Returns a boolean which indicates
# whether any assigned entry
# within the specified 3x3 box
# matches the given number
def used_in_box(arr, row, col, num):
    for i in range(3):
        for j in range(3):
            if(arr[i + row][j + col] == num):
                return True
    return False
 
# Checks whether it will be legal
# to assign num to the given row, col
# Returns a boolean which indicates
# whether it will be legal to assign
# num to the given row, col location.
def check_location_is_safe(arr, row, col, num):
     
    # Check if 'num' is not already
    # placed in current row,
    # current column and current 3x3 box
    return (not used_in_row(arr, row, num) and
           (not used_in_col(arr, col, num) and
           (not used_in_box(arr, row - row % 3,
                           col - col % 3, num))))
 
# Takes a partially filled-in grid
# and attempts to assign values to
# all unassigned locations in such a
# way to meet the requirements
# for Sudoku solution (non-duplication
# across rows, columns, and boxes)
def solve_sudoku(arr):
     
    # 'l' is a list variable that keeps the
    # record of row and col in
    # find_empty_location Function   
    l =[0, 0]
     
    # If there is no unassigned
    # location, we are done   
    if(not find_empty_location(arr, l)):
        return True
     
    # Assigning list values to row and col
    # that we got from the above Function
    row = l[0]
    col = l[1]
     
    # consider digits 1 to 9
    for num in range(1, 10):
         
        # if looks promising
        if(check_location_is_safe(arr,
                          row, col, num)):
             
            # make tentative assignment
            arr[row][col]= num
 
            # return, if success,
            # ya !
            if(solve_sudoku(arr)):
                return True
 
            # failure, unmake & try again
            arr[row][col] = 0
             
    # this triggers backtracking       
    return False
 
# Driver main function to test above functions
if __name__=="__main__":
     
    # creating a 2D array for the grid
    grid =[[0 for x in range(9)]for y in range(9)]
     
    # assigning values to the grid
    grid =[[3, 0, 6, 5, 0, 8, 4, 0, 0],
          [5, 2, 0, 0, 0, 0, 0, 0, 0],
          [0, 8, 7, 0, 0, 0, 0, 3, 1],
          [0, 0, 3, 0, 1, 0, 0, 8, 0],
          [9, 0, 0, 8, 6, 3, 0, 0, 5],
          [0, 5, 0, 0, 9, 0, 6, 0, 0],
          [1, 3, 0, 0, 0, 0, 2, 5, 0],
          [0, 0, 0, 0, 0, 0, 0, 7, 4],
          [0, 0, 5, 2, 0, 6, 3, 0, 0]]
     
    # if success print the grid
    if(solve_sudoku(grid)):
        print_grid(grid)
    else:
        print ("No solution exists")
 
# The above code has been contributed by Harshit Sidhwa.


C#




/* A Backtracking program in
C# to solve Sudoku problem */
using System;
 
class GFG
{
 
    public static bool isSafe(int[, ] board,
                              int row, int col,
                              int num)
    {
         
        // Row has the unique (row-clash)
        for (int d = 0; d < board.GetLength(0); d++)
        {
             
            // Check if the number
            // we are trying to
            // place is already present in
            // that row, return false;
            if (board[row, d] == num)
            {
                return false;
            }
        }
 
        // Column has the unique numbers (column-clash)
        for (int r = 0; r < board.GetLength(0); r++)
        {
             
            // Check if the number
            // we are trying to
            // place is already present in
            // that column, return false;
            if (board[r, col] == num)
            {
                return false;
            }
        }
 
        // corresponding square has
        // unique number (box-clash)
        int sqrt = (int)Math.Sqrt(board.GetLength(0));
        int boxRowStart = row - row % sqrt;
        int boxColStart = col - col % sqrt;
 
        for (int r = boxRowStart;
             r < boxRowStart + sqrt; r++)
        {
            for (int d = boxColStart;
                 d < boxColStart + sqrt; d++)
            {
                if (board[r, d] == num)
                {
                    return false;
                }
            }
        }
 
        // if there is no clash, it's safe
        return true;
    }
 
    public static bool solveSudoku(int[, ] board,
                                           int n)
    {
        int row = -1;
        int col = -1;
        bool isEmpty = true;
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (board[i, j] == 0)
                {
                    row = i;
                    col = j;
 
                    // We still have some remaining
                    // missing values in Sudoku
                    isEmpty = false;
                    break;
                }
            }
            if (!isEmpty)
            {
                break;
            }
        }
 
        // no empty space left
        if (isEmpty)
        {
            return true;
        }
 
        // else for each-row backtrack
        for (int num = 1; num <= n; num++)
        {
            if (isSafe(board, row, col, num))
            {
                board[row, col] = num;
                if (solveSudoku(board, n))
                {
                     
                    // Print(board, n);
                    return true;
                }
                else
                {
                     
                    // Replace it
                    board[row, col] = 0;
                }
            }
        }
        return false;
    }
 
    public static void print(int[, ] board, int N)
    {
         
        // We got the answer, just print it
        for (int r = 0; r < N; r++)
        {
            for (int d = 0; d < N; d++)
            {
                Console.Write(board[r, d]);
                Console.Write(" ");
            }
            Console.Write("\n");
 
            if ((r + 1) % (int)Math.Sqrt(N) == 0)
            {
                Console.Write("");
            }
        }
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
 
        int[, ] board = new int[, ] {
            { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
            { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
            { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
            { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
            { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
            { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
            { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
            { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
            { 0, 0, 5, 2, 0, 6, 3, 0, 0 }
        };
        int N = board.GetLength(0);
 
        if (solveSudoku(board, N))
        {
             
            // print solution
            print(board, N);
        }
        else {
            Console.Write("No solution");
        }
    }
}
 
// This code has been contributed by 29AjayKumar


Javascript




<script>
 
/* A Backtracking program in
Javascript to solve Sudoku problem */
 
function isSafe(board, row, col, num)
{
     
    // Row has the unique (row-clash)
    for(let d = 0; d < board.length; d++)
    {
         
        // Check if the number we are trying to
        // place is already present in
        // that row, return false;
        if (board[row][d] == num)
        {
            return false;
        }
    }
 
    // Column has the unique numbers (column-clash)
    for(let r = 0; r < board.length; r++)
    {
          
        // Check if the number
        // we are trying to
        // place is already present in
        // that column, return false;
        if (board[r][col] == num)
        {
            return false;
        }
    }
 
    // Corresponding square has
    // unique number (box-clash)
    let sqrt = Math.floor(Math.sqrt(board.length));
    let boxRowStart = row - row % sqrt;
    let boxColStart = col - col % sqrt;
 
    for(let r = boxRowStart;
            r < boxRowStart + sqrt; r++)
    {
        for(let d = boxColStart;
                d < boxColStart + sqrt; d++)
        {
            if (board[r][d] == num)
            {
                return false;
            }
        }
    }
 
    // If there is no clash, it's safe
    return true;
}
 
function solveSudoku(board, n)
{
    let row = -1;
    let col = -1;
    let isEmpty = true;
    for(let i = 0; i < n; i++)
    {
        for(let j = 0; j < n; j++)
        {
            if (board[i][j] == 0)
            {
                row = i;
                col = j;
 
                // We still have some remaining
                // missing values in Sudoku
                isEmpty = false;
                break;
            }
        }
        if (!isEmpty)
        {
            break;
        }
    }
 
    // No empty space left
    if (isEmpty)
    {
        return true;
    }
 
    // Else for each-row backtrack
    for(let num = 1; num <= n; num++)
    {
        if (isSafe(board, row, col, num))
        {
            board[row][col] = num;
            if (solveSudoku(board, n))
            {
                 
                // print(board, n);
                return true;
            }
            else
            {
                 
                // Replace it
                board[row][col] = 0;
            }
        }
    }
    return false;
}
 
function print(board, N)
{
     
    // We got the answer, just print it
    for(let r = 0; r < N; r++)
    {
        for(let d = 0; d < N; d++)
        {
            document.write(board[r][d]);
            document.write(" ");
        }
        document.write("<br>");
 
        if ((r + 1) % Math.floor(Math.sqrt(N)) == 0)
        {
            document.write("");
        }
    }
}
 
// Driver Code
let board = [ [ 3, 0, 6, 5, 0, 8, 4, 0, 0 ],
              [ 5, 2, 0, 0, 0, 0, 0, 0, 0 ],
              [ 0, 8, 7, 0, 0, 0, 0, 3, 1 ],
              [ 0, 0, 3, 0, 1, 0, 0, 8, 0 ],
              [ 9, 0, 0, 8, 6, 3, 0, 0, 5 ],
              [ 0, 5, 0, 0, 9, 0, 6, 0, 0 ],
              [ 1, 3, 0, 0, 0, 0, 2, 5, 0 ],
              [ 0, 0, 0, 0, 0, 0, 0, 7, 4 ],
              [ 0, 0, 5, 2, 0, 6, 3, 0, 0 ] ];
         
let N = board.length;
 
if (solveSudoku(board, N))
{
     
    // Print solution
    print(board, N);
}
else
{
    document.write("No solution");
}
 
// This code is contributed by avanitrachhadiya2155
 
</script>


Output

3 1 6 5 7 8 4 9 2 
5 2 9 1 3 4 7 6 8 
4 8 7 6 2 9 5 3 1 
2 6 3 4 1 5 9 8 7 
9 7 4 8 6 3 1 2 5 
8 5 1 7 9 2 6 4 3 
1 3 8 9 4 7 2 5 6 
6 9 2 3 5 1 8 7 4 
7 4 5 2 8 6 3 1 9

Time complexity: O(9(N*N)), For every unassigned index, there are 9 possible options so the time complexity is O(9^(n*n)). The time complexity remains the same but there will be some early pruning so the time taken will be much less than the naive algorithm but the upper bound time complexity remains the same.
Space Complexity: O(N*N), To store the output array a matrix is needed.

Sudoku using Bit Masks:

This method is a slight optimization to the above 2 methods.  For each row/column/box create a bitmask and for each element in the grid set the bit at position ‘value’ to 1 in the corresponding bitmasks, for O(1) checks.

Follow the steps below to solve the problem:

  • Create 3 arrays of size N (one for rows, columns, and boxes).
  • The boxes are indexed from 0 to 8. (in order to find the box index of an element we use the following formula: row / 3 * 3 + column / 3).
  • Map the initial values of the grid first.
  • Each time we add/remove an element to/from the grid set the bit to 1/0 to the corresponding bitmasks.

Below is the implementation of the above approach:

C++




#include <bits/stdc++.h>
using namespace std;
 
#define N 9
 
// Bitmasks for each row/column/box
int row[N], col[N], box[N];
bool seted = false;
 
// Utility function to find the box index
// of an element at position [i][j] in the grid
int getBox(int i, int j) { return i / 3 * 3 + j / 3; }
 
// Utility function to check if a number
// is present in the corresponding row/column/box
bool isSafe(int i, int j, int number)
{
    return !((row[i] >> number) & 1)
           && !((col[j] >> number) & 1)
           && !((box[getBox(i, j)] >> number) & 1);
}
 
// Utility function to set the initial values of a Sudoku
// board (map the values in the bitmasks)
void setInitialValues(int grid[N][N])
{
    for (int i = 0; i < N; i++)
        for (int j = 0; j < N; j++)
            row[i] |= 1 << grid[i][j],
                col[j] |= 1 << grid[i][j],
                box[getBox(i, j)] |= 1 << grid[i][j];
}
 
/* Takes a partially filled-in grid and attempts
to assign values to all unassigned locations in
such a way to meet the requirements for
Sudoku solution (non-duplication across rows,
columns, and boxes) */
bool SolveSudoku(int grid[N][N], int i, int j)
{
    // Set the initial values
    if (!seted)
        seted = true, setInitialValues(grid);
 
    if (i == N - 1 && j == N)
        return true;
    if (j == N)
        j = 0, i++;
 
    if (grid[i][j])
        return SolveSudoku(grid, i, j + 1);
 
    for (int nr = 1; nr <= N; nr++) {
        if (isSafe(i, j, nr)) {
            /*  Assign nr in the
                current (i, j)
                position and
                add nr to each bitmask
            */
            grid[i][j] = nr;
            row[i] |= 1 << nr;
            col[j] |= 1 << nr;
            box[getBox(i, j)] |= 1 << nr;
 
            if (SolveSudoku(grid, i, j + 1))
                return true;
 
            // Remove nr from each bitmask
            // and search for another possibility
            row[i] &= ~(1 << nr);
            col[j] &= ~(1 << nr);
            box[getBox(i, j)] &= ~(1 << nr);
        }
 
        grid[i][j] = 0;
    }
 
    return false;
}
 
// Utility function to print the solved grid
void print(int grid[N][N])
{
    for (int i = 0; i < N; i++, cout << '\n')
        for (int j = 0; j < N; j++)
            cout << grid[i][j] << ' ';
}
 
// Driver Code
int main()
{
    // 0 means unassigned cells
    int grid[N][N] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                       { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                       { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                       { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                       { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                       { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                       { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                       { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                       { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
    if (SolveSudoku(grid, 0, 0))
        print(grid);
    else
        cout << "No solution exists\n";
 
    return 0;
}
// This code is contributed
// by Gatea David


Java




/*package whatever //do not write package name here */
import java.io.*;
 
class GFG {
    static int N = 9;
 
    // Bitmasks for each row/column/box
    static int row[] = new int[N], col[] = new int[N],
               box[] = new int[N];
    static Boolean seted = false;
 
    // Utility function to find the box index
    // of an element at position [i][j] in the grid
    static int getBox(int i, int j)
    {
        return i / 3 * 3 + j / 3;
    }
 
    // Utility function to check if a number
    // is present in the corresponding row/column/box
    static Boolean isSafe(int i, int j, int number)
    {
        return ((row[i] >> number) & 1) == 0
            && ((col[j] >> number) & 1) == 0
            && ((box[getBox(i, j)] >> number) & 1) == 0;
    }
 
    // Utility function to set the initial values of a
    // Sudoku board (map the values in the bitmasks)
    static void setInitialValues(int grid[][])
    {
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid.length; j++) {
                row[i] |= 1 << grid[i][j];
                col[j] |= 1 << grid[i][j];
                box[getBox(i, j)] |= 1 << grid[i][j];
            }
        }
    }
 
    /* Takes a partially filled-in grid and attempts
      to assign values to all unassigned locations in
      such a way to meet the requirements for
      Sudoku solution (non-duplication across rows,
      columns, and boxes) */
    static Boolean SolveSudoku(int grid[][], int i, int j)
    {
        // Set the initial values
        if (!seted) {
            seted = true;
            setInitialValues(grid);
        }
 
        if (i == N - 1 && j == N)
            return true;
        if (j == N) {
            j = 0;
            i++;
        }
 
        if (grid[i][j] > 0)
            return SolveSudoku(grid, i, j + 1);
 
        for (int nr = 1; nr <= N; nr++) {
            if (isSafe(i, j, nr)) {
                /* Assign nr in the
                            current (i, j)
                            position and
                            add nr to each bitmask
                        */
                grid[i][j] = nr;
                row[i] |= 1 << nr;
                col[j] |= 1 << nr;
                box[getBox(i, j)] |= 1 << nr;
 
                if (SolveSudoku(grid, i, j + 1))
                    return true;
 
                // Remove nr from each bitmask
                // and search for another possibility
                row[i] &= ~(1 << nr);
                col[j] &= ~(1 << nr);
                box[getBox(i, j)] &= ~(1 << nr);
            }
 
            grid[i][j] = 0;
        }
 
        return false;
    }
 
    // Utility function to print the solved grid
    static void print(int grid[][])
    {
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[0].length; j++) {
                System.out.printf("%d ", grid[i][j]);
            }
            System.out.println();
        }
    }
 
    // Driver code
    public static void main(String args[])
    {
        // 0 means unassigned cells
        int grid[][] = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                         { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                         { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                         { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                         { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                         { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                         { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                         { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                         { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
        if (SolveSudoku(grid, 0, 0))
            print(grid);
        else
            System.out.println("No solution exists");
    }
}
 
// This code is contributed by shinjanpatra.


Python3




# N is the size of the 2D matrix N*N
N = 9
 
# A utility function to print grid
 
 
def printing(arr):
    for i in range(N):
        for j in range(N):
            print(arr[i][j], end=" ")
        print()
 
# Checks whether it will be
# legal to assign num to the
# given row, col
 
 
def isSafe(grid, row, col, num):
 
    # Check if we find the same num
    # in the similar row , we
    # return false
    for x in range(9):
        if grid[row][x] == num:
            return False
 
    # Check if we find the same num in
    # the similar column , we
    # return false
    for x in range(9):
        if grid[x][col] == num:
            return False
 
    # Check if we find the same num in
    # the particular 3*3 matrix,
    # we return false
    startRow = row - row % 3
    startCol = col - col % 3
    for i in range(3):
        for j in range(3):
            if grid[i + startRow][j + startCol] == num:
                return False
    return True
 
# Takes a partially filled-in grid and attempts
# to assign values to all unassigned locations in
# such a way to meet the requirements for
# Sudoku solution (non-duplication across rows,
# columns, and boxes) */
 
 
def solveSudoku(grid, row, col):
 
    # Check if we have reached the 8th
    # row and 9th column (0
    # indexed matrix) , we are
    # returning true to avoid
    # further backtracking
    if (row == N - 1 and col == N):
        return True
 
    # Check if column value becomes 9 ,
    # we move to next row and
    # column start from 0
    if col == N:
        row += 1
        col = 0
 
    # Check if the current position of
    # the grid already contains
    # value >0, we iterate for next column
    if grid[row][col] > 0:
        return solveSudoku(grid, row, col + 1)
    for num in range(1, N + 1, 1):
 
        # Check if it is safe to place
        # the num (1-9) in the
        # given row ,col ->we
        # move to next column
        if isSafe(grid, row, col, num):
 
            # Assigning the num in
            # the current (row,col)
            # position of the grid
            # and assuming our assigned
            # num in the position
            # is correct
            grid[row][col] = num
 
            # Checking for next possibility with next
            # column
            if solveSudoku(grid, row, col + 1):
                return True
 
        # Removing the assigned num ,
        # since our assumption
        # was wrong , and we go for
        # next assumption with
        # diff num value
        grid[row][col] = 0
    return False
 
# Driver Code
 
 
# 0 means unassigned cells
grid = [[3, 0, 6, 5, 0, 8, 4, 0, 0],
        [5, 2, 0, 0, 0, 0, 0, 0, 0],
        [0, 8, 7, 0, 0, 0, 0, 3, 1],
        [0, 0, 3, 0, 1, 0, 0, 8, 0],
        [9, 0, 0, 8, 6, 3, 0, 0, 5],
        [0, 5, 0, 0, 9, 0, 6, 0, 0],
        [1, 3, 0, 0, 0, 0, 2, 5, 0],
        [0, 0, 0, 0, 0, 0, 0, 7, 4],
        [0, 0, 5, 2, 0, 6, 3, 0, 0]]
 
if (solveSudoku(grid, 0, 0)):
    printing(grid)
else:
    print("no solution exists ")
 
# This code is contributed by sanjoy_62.


C#




// C# program for above approach
using System;
class GFG {
 
    // N is the size of the 2D matrix   N*N
    static int N = 9;
 
    /* Takes a partially filled-in grid and attempts
      to assign values to all unassigned locations in
      such a way to meet the requirements for
      Sudoku solution (non-duplication across rows,
      columns, and boxes) */
    static bool solveSudoku(int[, ] grid, int row, int col)
    {
 
        /*if we have reached the 8th
               row and 9th column (0
               indexed matrix) ,
               we are returning true to avoid further
               backtracking       */
        if (row == N - 1 && col == N)
            return true;
 
        // Check if column value  becomes 9 ,
        // we move to next row
        // and column start from 0
        if (col == N) {
            row++;
            col = 0;
        }
 
        // Check if the current position
        // of the grid already
        // contains value >0, we iterate
        // for next column
        if (grid[row, col] != 0)
            return solveSudoku(grid, row, col + 1);
 
        for (int num = 1; num < 10; num++) {
 
            // Check if it is safe to place
            // the num (1-9)  in the
            // given row ,col ->we move to next column
            if (isSafe(grid, row, col, num)) {
 
                /*  assigning the num in the current
                        (row,col)  position of the grid and
                        assuming our assigned num in the
                   position is correct */
                grid[row, col] = num;
 
                // Checking for next
                // possibility with next column
                if (solveSudoku(grid, row, col + 1))
                    return true;
            }
            /* removing the assigned num , since our
                     assumption was wrong , and we go for
               next assumption with diff num value   */
            grid[row, col] = 0;
        }
        return false;
    }
 
    /* A utility function to print grid */
    static void print(int[, ] grid)
    {
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++)
                Console.Write(grid[i, j] + " ");
            Console.WriteLine();
        }
    }
 
    // Check whether it will be legal
    // to assign num to the
    // given row, col
    static bool isSafe(int[, ] grid, int row, int col,
                       int num)
    {
 
        // Check if we find the same num
        // in the similar row , we
        // return false
        for (int x = 0; x <= 8; x++)
            if (grid[row, x] == num)
                return false;
 
        // Check if we find the same num
        // in the similar column ,
        // we return false
        for (int x = 0; x <= 8; x++)
            if (grid[x, col] == num)
                return false;
 
        // Check if we find the same num
        // in the particular 3*3
        // matrix, we return false
        int startRow = row - row % 3, startCol
                                      = col - col % 3;
        for (int i = 0; i < 3; i++)
            for (int j = 0; j < 3; j++)
                if (grid[i + startRow, j + startCol] == num)
                    return false;
 
        return true;
    }
 
    // Driver code
    static void Main()
    {
        int[, ] grid = { { 3, 0, 6, 5, 0, 8, 4, 0, 0 },
                         { 5, 2, 0, 0, 0, 0, 0, 0, 0 },
                         { 0, 8, 7, 0, 0, 0, 0, 3, 1 },
                         { 0, 0, 3, 0, 1, 0, 0, 8, 0 },
                         { 9, 0, 0, 8, 6, 3, 0, 0, 5 },
                         { 0, 5, 0, 0, 9, 0, 6, 0, 0 },
                         { 1, 3, 0, 0, 0, 0, 2, 5, 0 },
                         { 0, 0, 0, 0, 0, 0, 0, 7, 4 },
                         { 0, 0, 5, 2, 0, 6, 3, 0, 0 } };
 
        if (solveSudoku(grid, 0, 0))
            print(grid);
        else
            Console.WriteLine("No Solution exists");
    }
}
 
// This code is contributed by code_hunt.


Javascript




<script>
 
const N = 9
 
// Bitmasks for each row/column/box
let row = new Array(N), col = new Array(N), box = new Array(N);
let seted = false;
 
// Utility function to find the box index
// of an element at position [i][j] in the grid
function getBox(i,j)
{
    return Math.floor(i / 3) * 3 + Math.floor(j / 3);
}
 
// Utility function to check if a number
// is present in the corresponding row/column/box
function isSafe(i,j,number)
{
    return !((row[i] >> number) & 1)
        && !((col[j] >> number) & 1)
        && !((box[getBox(i,j)] >> number) & 1);
}
 
// Utility function to set the initial values of a Sudoku board
// (map the values in the bitmasks)
function setInitialValues(grid)
{
    for (let i = 0; i < N;i++)
        for (let j = 0; j < N; j++)
                row[i] |= 1 << grid[i][j],
                col[j] |= 1 << grid[i][j],
                box[getBox(i, j)] |= 1 << grid[i][j];
}
 
/* Takes a partially filled-in grid and attempts
to assign values to all unassigned locations in
such a way to meet the requirements for
Sudoku solution (non-duplication across rows,
columns, and boxes) */
function SolveSudoku(grid ,i, j)
{
    // Set the initial values
    if(!seted){
        seted = true,
        setInitialValues(grid);
    }
 
    if(i == N - 1 && j == N)
        return true;
    if(j == N){
        j = 0;
        i++;
    }
 
    if(grid[i][j])
        return SolveSudoku(grid, i, j + 1);
 
    for (let nr = 1; nr <= N;nr++)
    {
        if(isSafe(i, j, nr))
        {
            /* Assign nr in the
                current (i, j)
                position and
                add nr to each bitmask
            */
            grid[i][j] = nr;
            row[i] |= 1 << nr;
            col[j] |= 1 << nr;
            box[getBox(i, j)] |= 1 << nr;
 
            if(SolveSudoku(grid, i,j + 1))
                return true;
 
            // Remove nr from each bitmask
            // and search for another possibility
            row[i] &= ~(1 << nr);
            col[j] &= ~(1 << nr);
            box[getBox(i, j)] &= ~(1 << nr);
        }
 
        grid[i][j] = 0;
    }
 
    return false;
}
 
// Utility function to print the solved grid
function print(grid)
{
    for (let i = 0; i < N; i++){
        for (let j = 0; j < N; j++){
             document.write(grid[i][j]," ");
        }
        document.write("</br>");
    }
}
 
// Driver Code
 
    // 0 means unassigned cells
    let grid = [ [ 3, 0, 6, 5, 0, 8, 4, 0, 0 ],
                    [ 5, 2, 0, 0, 0, 0, 0, 0, 0 ],
                    [ 0, 8, 7, 0, 0, 0, 0, 3, 1 ],
                    [ 0, 0, 3, 0, 1, 0, 0, 8, 0 ],
                    [ 9, 0, 0, 8, 6, 3, 0, 0, 5 ],
                    [ 0, 5, 0, 0, 9, 0, 6, 0, 0 ],
                    [ 1, 3, 0, 0, 0, 0, 2, 5, 0 ],
                    [ 0, 0, 0, 0, 0, 0, 0, 7, 4 ],
                    [ 0, 0, 5, 2, 0, 6, 3, 0, 0 ]];
 
    if (SolveSudoku(grid,0 ,0))
        print(grid);
    else
        document.write("No solution exists","</br>");
 
// This code is contributed by shinjanpatra
 
</script>


Output

3 1 6 5 7 8 4 9 2 
5 2 9 1 3 4 7 6 8 
4 8 7 6 2 9 5 3 1 
2 6 3 4 1 5 9 8 7 
9 7 4 8 6 3 1 2 5 
8 5 1 7 9 2 6 4 3 
1 3 8 9 4 7 2 5 6 
6 9 2 3 5 1 8 7 4 
7 4 5 2 8 6 3 1 9 

Time complexity: O(9(N*N)). For every unassigned index, there are 9 possible options so the time complexity is O(9^(n*n)). The time complexity remains the same but checking if a number is safe to use is much faster, O(1).
Space Complexity: O(N*N). To store the output array a matrix is needed, and 3 extra arrays of size n are needed for the bitmasks.

Sudoku using Cross-Hatching with backtracking:

This method is an optimization of the above method 2. It runs 5X times faster than method 2. Like we used to fill sudoku by first identifying the element which is almost filled. It starts with identifying the row and column where the element should be placed. Picking the almost-filled elements first will give better pruning.

Follow the steps below to solve the problem:

  1. Build a graph with pending elements mapped to row and column coordinates where they can be fitted in the original matrix.
  2. Pick the elements from the graph sorted by fewer remaining elements to be filled.
  3. Recursively fill the elements using a graph into the matrix. Backtrack once an unsafe position is discovered.

Below is the implementation of the above approach:

Python3




# This program works by identifying the remaining elements and backtrack only on thoese.
# The elements are inserted in the increasing order of the elements left to be inserted. And hence runs much faster.
# Comparing with other back tracking algorithms, it runs 5X faster.
 
# Input matrix
arr = [
    [3, 0, 6, 5, 0, 8, 4, 0, 0],
    [5, 2, 0, 0, 0, 0, 0, 0, 0],
    [0, 8, 7, 0, 0, 0, 0, 3, 1],
    [0, 0, 3, 0, 1, 0, 0, 8, 0],
    [9, 0, 0, 8, 6, 3, 0, 0, 5],
    [0, 5, 0, 0, 9, 0, 6, 0, 0],
    [1, 3, 0, 0, 0, 0, 2, 5, 0],
    [0, 0, 0, 0, 0, 0, 0, 7, 4],
    [0, 0, 5, 2, 0, 6, 3, 0, 0]
]
 
# Position of the input elements in the arr
# pos = {
#     element: [[position 1], [position 2]]
# }
pos = {}
 
# Count of the remaining number of the elements
# rem = {
#     element: pending count
# }
rem = {}
 
# Graph defining tentative positions of the elements to be filled
# graph = {
#     key: {
#         row1: [columns],
#         row2: [columns]
#     }
# }
graph = {}
 
 
# Print the matrix array
def printMatrix():
    for i in range(0, 9):
        for j in range(0, 9):
            print(str(arr[i][j]), end=" ")
        print()
 
 
# Method to check if the inserted element is safe
def is_safe(x, y):
    key = arr[x][y]
    for i in range(0, 9):
        if i != y and arr[x][i] == key:
            return False
        if i != x and arr[i][y] == key:
            return False
 
    r_start = int(x / 3) * 3
    r_end = r_start + 3
 
    c_start = int(y / 3) * 3
    c_end = c_start + 3
 
    for i in range(r_start, r_end):
        for j in range(c_start, c_end):
            if i != x and j != y and arr[i][j] == key:
                return False
    return True
 
 
# method to fill the matrix
# input keys: list of elements to be filled in the matrix
#        k   : index number of the element to be picked up from keys
#        rows: list of row index where element is to be inserted
#        r   : index number of the row to be inserted
#
def fill_matrix(k, keys, r, rows):
    for c in graph[keys[k]][rows[r]]:
        if arr[rows[r]] > 0:
            continue
        arr[rows[r]] = keys[k]
        if is_safe(rows[r], c):
            if r < len(rows) - 1:
                if fill_matrix(k, keys, r + 1, rows):
                    return True
                else:
                    arr[rows[r]] = 0
                    continue
            else:
                if k < len(keys) - 1:
                    if fill_matrix(k + 1, keys, 0, list(graph[keys[k + 1]].keys())):
                        return True
                    else:
                        arr[rows[r]] = 0
                        continue
                return True
        arr[rows[r]] = 0
    return False
 
 
# Fill the pos and rem dictionary. It will be used to build graph
def build_pos_and_rem():
    for i in range(0, 9):
        for j in range(0, 9):
            if arr[i][j] > 0:
                if arr[i][j] not in pos:
                    pos[arr[i][j]] = []
                pos[arr[i][j]].append([i, j])
                if arr[i][j] not in rem:
                    rem[arr[i][j]] = 9
                rem[arr[i][j]] -= 1
 
    # Fill the elements not present in input matrix. Example: 1 is missing in input matrix
    for i in range(1, 10):
        if i not in pos:
            pos[i] = []
        if i not in rem:
            rem[i] = 9
 
# Build the graph
 
 
def build_graph():
    for k, v in pos.items():
        if k not in graph:
            graph[k] = {}
 
        row = list(range(0, 9))
        col = list(range(0, 9))
 
        for cord in v:
            row.remove(cord[0])
            col.remove(cord[1])
 
        if len(row) == 0 or len(col) == 0:
            continue
 
        for r in row:
            for c in col:
                if arr[r] == 0:
                    if r not in graph[k]:
                        graph[k][r] = []
                    graph[k][r].append(c)
 
 
build_pos_and_rem()
 
# Sort the rem map in order to start with smaller number of elements to be filled first. Optimization for pruning
rem = {k: v for k, v in sorted(rem.items(), key=lambda item: item[1])}
 
build_graph()
 
key_s = list(rem.keys())
# Util called to fill the matrix
fill_matrix(0, key_s, 0, list(graph[key_s[0]].keys()))
 
printMatrix()
 
# This code is contributed by Arun Kumar


Output

3 1 6 5 7 8 4 9 2 
5 2 9 1 3 4 7 6 8 
4 8 7 6 2 9 5 3 1 
2 6 3 4 1 5 9 8 7 
9 7 4 8 6 3 1 2 5 
8 5 1 7 9 2 6 4 3 
1 3 8 9 4 7 2 5 6 
6 9 2 3 5 1 8 7 4 
7 4 5 2 8 6 3 1 9 

Time complexity: O(9^(n*n)).  Due to the element that needs to fit in a cell will come earlier as we are filling almost filled elements first, it will help in less number of recursive calls. So the time taken will be much less than the naive approaches but the upper bound time complexity remains the same.
Auxiliary Space: O(n*n).  A graph of the remaining positions to be filled for the respected elements is created.


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