Kth character after replacing each character of String by its frequency exactly X times

Given a string S consisting of N digits from [1, 9] and a positive integer K and X. Every day each character of the string is replaced by its frequency and value. The task is to find the Kth character of the string after X days.

Examples:

Input: S = “1214”, K = 10, X = 3
Output: 4
Explanation:
1st day = “12214444”
2nd day = “1222214444444444444444”
3rd day = “122222222444444444444444444444444444444444444444444444444” 
So, 10th character after 3rd day is 4.

Input: S =”123″, K = 6, X = 2 
Output: 3

Naive Approach: The simplest approach to solve the problem is to create the string by appending the digits in the string digit^X times and finding the Kth character of the string.

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to find the Kth character
// after X days
char FindKthChar(string str, long long K, int X)
{
    string s = str;

    for (int i = 0; i < X; i++) {
        string res = "";
        for (int j = 0; j < s.size(); j++) {
            int count = s[j] - '0';
            while (count--) {
                res += s[j];
            }
        }
        s = res;
    }

    return s[K - 1];
}

// Driver Code
int main()
{
    // Given Input
    string str = "1214";
    long long K = 10;
    int X = 3;

    // Function Call
    char ans = FindKthChar(str, K, X);
    cout << ans << "\n";
    return 0;
}

Java

import java.util.*;

class Gfg {
    // Function to find the Kth character
    // after X days
    static char findKthChar(String str, long K, int X)
    {
        String s = str;

        for (int i = 0; i < X; i++) {
            String res = "";
            for (int j = 0; j < s.length(); j++) {
                int count = s.charAt(j) - '0';
                while (count-- > 0) {
                    res += s.charAt(j);
                }
            }
            s = res;
        }

        return s.charAt((int)(K - 1));
    }

    public static void main(String[] args)
    {
        // Given Input
        String str = "1214";
        long K = 10;
        int X = 3;

        // Function Call
        char ans = findKthChar(str, K, X);
        System.out.println(ans);
    }
}

C#

using System;

class Gfg
{

  // Function to find the Kth character
  // after X days
  static char findKthChar(string str, long K, int X)
  {
    string s = str;

    for (int i = 0; i < X; i++) {
      string res = "";
      for (int j = 0; j < s.Length; j++) {
        int count = s[j] - '0';
        while (count-- > 0) {
          res += s[j];
        }
      }
      s = res;
    }

    return s[(int)(K - 1)];
  }

  public static void Main(string[] args)
  {
    // Given Input
    string str = "1214";
    long K = 10;
    int X = 3;

    // Function Call
    char ans = findKthChar(str, K, X);
    Console.WriteLine(ans);
  }
}

// This code is contributed by divya_p123.

Output

1

Time Complexity: O(∑digit[i]^X) for i in range [0, N-1]
Auxiliary Space: O(∑digit[i]^X) for i in range [0, N-1]

Efficient Approach: The above approach can be optimized further by instead of creating a string add digit^X to the sum and check if sum > K. Follow the steps below to solve the problem:

• Initialize a variable, say sum that stores the sum of the character of the string after X days.
• Initialize a variable, say ans that stores the Kth characters after X days.
• Iterate in the range [0, N-1] and perform the following steps:
  • Initialize a variable say range as (S[i] – ‘0’)^X and add it to the variable sum.
  • If sum>=K, return S, modify ans as S[i], and terminate the loop.
• Print the value of ans as the answer.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to find the Kth character
// after X days
char FindKthChar(string str, long long K, int X)
{

    // Variable to store the KthChar
    char ans;
    int sum = 0;

    // Traverse the string
    for (int i = 0; i < str.length(); i++) {

        // Convert char into int
        int digit = str[i] - '0';

        // Calculate characters
        int range = pow(digit, X);
        sum += range;

        // If K is less than sum
        // than ans = str[i]
        if (K <= sum) {
            ans = str[i];
            break;
        }
    }
    // Return answer
    return ans;
}

// Driver Code
int main()
{
    // Given Input
    string str = "123";
    long long K = 9;
    int X = 3;

    // Function Call
    char ans = FindKthChar(str, K, X);
    cout << ans << "\n";
    return 0;
}

Java

// Java program for the above approach
class GFG{

// Function to find the Kth character
// after X days
static char FindKthChar(String str, int K, int X)
{

    // Variable to store the KthChar
    char ans = ' ';
    int sum = 0;

    // Traverse the string
    for(int i = 0; i < str.length(); i++)
    {

        // Convert char into int
        int digit = (int)str.charAt(i) - 48;

        // Calculate characters
        int range = (int)Math.pow(digit, X);
        sum += range;

        // If K is less than sum
        // than ans = str[i]
        if (K <= sum)
        {
            ans = str.charAt(i);
            break;
        }
    }

    // Return answer
    return ans;
}

// Driver code
public static void main(String[] args)
{

    // Given Input
    String str = "123";
    int K = 9;
    int X = 3;

    // Function Call
    char ans = FindKthChar(str, K, X);
    System.out.println(ans);
}
}

// This code is contributed by abhinavjain194

Python3

# Python3 program for the above approach
import math

# Function to find the Kth character
# after X days
def FindKthChar(Str, K, X):

    # Variable to store the KthChar
    ans = ' '
    Sum = 0

    # Traverse the string
    for i in range(len(Str)):

        # Convert char into int
        digit = ord(Str[i]) - 48

        # Calculate characters
        Range = int(math.pow(digit, X))
        Sum += Range

        # If K is less than sum
        # than ans = str[i]
        if (K <= Sum):
            ans = Str[i]
            break

    # Return answer
    return ans

# Given Input
Str = "123"
K = 9
X = 3

# Function Call
ans = FindKthChar(Str, K, X)
print(ans)

# This code is contributed by divyeshrabadiya07.

C#

// C# program for the above approach
using System;
using System.Collections.Generic;

class GFG{

// Function to find the Kth character
// after X days
static char FindKthChar(string str, int K, int X)
{

    // Variable to store the KthChar
    char ans = ' ';
    int sum = 0;

    // Traverse the string
    for(int i = 0; i < str.Length; i++)
    {

        // Convert char into int
        int digit = (int)str[i] - 48;

        // Calculate characters
        int range = (int)Math.Pow(digit, X);
        sum += range;

        // If K is less than sum
        // than ans = str[i]
        if (K <= sum)
        {
            ans = str[i];
            break;
        }
    }

    // Return answer
    return ans;
}

// Driver Code
public static void Main()
{

    // Given Input
    string str = "123";
    int K = 9;
    int X = 3;

    // Function Call
    char ans = FindKthChar(str, K, X);
    Console.Write(ans);
}
}

// This code is contributed by SURENDRA_GANGWAR

Javascript

<script>
// javascript program for the above approach
// Function to find the Kth character
    // after X days
     function FindKthChar( str , K , X) {

        // Variable to store the KthChar
        var ans = "";
        var sum = 0;

        // Traverse the string
        for (i = 0; i < str.length; i++) {

            // Convert char into int
            var digit = parseInt( str[i]);

            // Calculate characters
            var range = parseInt( Math.pow(digit, X));
            sum += range;

            // If K is less than sum
            // than ans = str[i]
            if (K <= sum) {
                ans = str[i];
                break;
            }
        }

        // Return answer
        return ans;
    }

    // Driver code


        // Given Input
        var str = "123";
        var K = 9;
        var X = 3;

        // Function Call
        var ans = FindKthChar(str, K, X);
        document.write(ans);

// This code contributed by gauravrajput1
</script>

Output

2

Time Complexity: O(N), since there is only one loop to carry out the operations the overall complexity turns out to be O(N)
Auxiliary Space: O(1), since there is no extra array or data structure used, it takes constant space.