# Lexicographically largest permutation by sequentially inserting Array elements at ends

• Difficulty Level : Easy
• Last Updated : 28 Feb, 2022

Given an array arr[] of N integers, the task is to find the lexicographically largest permutation by sequentially inserting the array elements to the front or the back of another array.

Examples:

Input: arr[] = {3, 1, 2, 4}
Output: 4 3 1 2
Explanation:
The permutations that can be created by sequentially inserting the array elements to the front or the back of the container are {3, 1, 2, 4}, {1, 3, 2, 4}, {2, 3, 1, 4}, {2, 1, 3, 4}, {4, 1, 3, 2}, {4, 2, 3, 1}, {4, 2, 1, 3}, and {4, 3, 1, 2}. Out of which {4, 3, 1, 2} is the lexicographically largest permutation.

Input: arr[] = {1, 2, 3, 4, 5}
Output: 5 4 3 2 1

Approach: The given problem can be solved by using the Greedy Approach using the deque which is based on the observation that if the current array element is at least the first element of the new array, the most optimal choice always is to insert that element in front of the container in order to lexicographically maximize the permutation. Otherwise, insert the element to the end of the array. Follow the steps below to solve the given problem:

• Initialize a deque, say DQ, which stores the current state of the container.
• Initialize a variable, say mx, which stores the maximum till each index representing the 1st element of the deque DQ.
• Traverse the given array arr[] and if the current element arr[i] >= mx, then insert arr[i] to the front of the deque DQ and update the value of mx. Otherwise, insert arr[i] to the back of the deque DQ.
• After completing the above steps, print the elements stored in the deque DQ as the resultant largest permutation.

Below is the implementation of the above approach:

## C++

 // C++ program for the above approach   #include using namespace std;   // Function to find the lexicographically // largest permutation by sequentially // inserting the array elements void largestPermutation(int arr[], int N) {       // Stores the current state of     // the new array     deque p;       // Stores the current maximum     // element of array arr[]     int mx = arr[0];     p.push_back(arr[0]);       // Iterate the array elements     for (int i = 1; i < N; i++) {           // If the current element is         // smaller than the current         // maximum, then insert         if (arr[i] < mx)             p.push_back(arr[i]);           // If the current element is         // at least the current maximum         else {             p.push_front(arr[i]);               // Update the value of             // the current maximum             mx = arr[i];         }     }       // Print resultant permutation     for (auto i : p)         cout << i << " "; }   // Driver Code int main() {     int arr[] = { 3, 1, 2, 4 };     int N = sizeof(arr) / sizeof(arr[0]);       largestPermutation(arr, N);       return 0; }

## Java

 // Java code for the above approach import java.util.*;   class GFG {       // Function to find the lexicographically     // largest permutation by sequentially     // inserting the array elements     static void largestPermutation(int arr[], int N)     {           // Stores the current state of         // the new array         Deque p = new LinkedList();           // Stores the current maximum         // element of array arr[]         int mx = arr[0];         p.addLast(arr[0]);           // Iterate the array elements         for (int i = 1; i < N; i++) {               // If the current element is             // smaller than the current             // maximum, then insert             if (arr[i] < mx)                 p.addLast(arr[i]);               // If the current element is             // at least the current maximum             else {                 p.addFirst(arr[i]);                   // Update the value of                 // the current maximum                 mx = arr[i];             }         }           // Print resultant permutation         for (Iterator itr = p.iterator(); itr.hasNext();) {             System.out.print(itr.next() + " ");         }     }       // Driver Code       public static void main(String[] args)     {         int arr[] = { 3, 1, 2, 4 };         int N = arr.length;           largestPermutation(arr, N);     } }   // This code is contributed by Potta Lokesh

## Python3

 # python program for the above approach   # Function to find the lexicographically # largest permutation by sequentially # inserting the array elements from typing import Deque   def largestPermutation(arr, N):       # Stores the current state of     # the new array     p = Deque()       # Stores the current maximum     # element of array arr[]     mx = arr[0]     p.append(arr[0])       # Iterate the array elements     for i in range(1, N):           # If the current element is         # smaller than the current         # maximum, then insert         if (arr[i] < mx):             p.append(arr[i])           # If the current element is         # at least the current maximum         else:             p.appendleft(arr[i])               # Update the value of             # the current maximum             mx = arr[i]       # Print resultant permutation     for i in p:         print(i, end=" ")   # Driver Code if __name__ == "__main__":       arr = [3, 1, 2, 4]     N = len(arr)       largestPermutation(arr, N)   # This code is contributed by rakeshsahni

## C#

 // C# code for the above approach using System; using System.Linq; using System.Collections.Generic; public class GFG {     // Function to find the lexicographically   // largest permutation by sequentially   // inserting the array elements   static void largestPermutation(int []arr, int N) {       // Stores the current state of     // the new array     List p = new List();       // Stores the current maximum     // element of array []arr     int mx = arr[0];     p.Add(arr[0]);       // Iterate the array elements     for (int i = 1; i < N; i++) {         // If the current element is       // smaller than the current       // maximum, then insert       if (arr[i] < mx)         p.Add(arr[i]);         // If the current element is       // at least the current maximum       else {         p.Insert(0,arr[i]);           // Update the value of         // the current maximum         mx = arr[i];       }     }       // Print resultant permutation     foreach (int itr in p) {       Console.Write(itr + " ");     }   }     // Driver Code   public static void Main(String[] args) {     int []arr = { 3, 1, 2, 4 };     int N = arr.Length;       largestPermutation(arr, N);   } }   // This code is contributed by Rajput-Ji

## Javascript



Output:

4 3 1 2

Time Complexity: O(N)
Auxiliary Space: O(N)

My Personal Notes arrow_drop_up
Related Articles