Sort an array according to the increasing frequency of the digit K in the array elements

Last Updated : 23 Jul, 2025

Given an array arr[] of size N, and an integer K representing a digit, the task is to print the given array in increasing order according to the increasing frequency of the digit K in the array elements.

Examples:

Input: arr[] = {15, 66, 26, 91}, K = 6
Output: 15 91 26 66
Explanation:
The frequency of digit 6 in array elements is {0, 2, 1, 0}. The elements in increasing order of frequency of the digit 6 in {15, 91, 26, 66}.

Input: arr[] = {20, 21, 0}, K = 0
Output: 21 20 0
Explanation:
The frequency of digit 0 in array elements is {1, 0, 1}. The elements in increasing order of frequency of the digit 0 is {21, 20, 0}.

Approach: The idea is to count the occurrences of the digit K for each element of the array and sort the array according to it. Follow the steps below to solve the problem:

Initialize a multimap, say mp, to store occurrences of K in each element in sorted order.
Traverse the given array arr[] using the variable i and perform the following:
- Store the occurrences of digit K in arr[i] in a variable cnt.
- Insert a pair of cnt and arr[i] in mp.
Print the array elements in mp to get the required sorted order.

Below is the implementation of the above approach:

C++

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

// Function to count the occurrences
// of digit K in an element
int countOccurrences(int num, int K)
{
    // If num and K are both 0
    if (K == 0 && num == 0)
        return 1;

    // Initialize count as 0
    int count = 0;

    // Count for occurrences of digit K
    while (num > 0) {
        if (num % 10 == K)
            count++;
        num /= 10;
    }

    // Return the count
    return count;
}

// Function to print the given array
// in increasing order of the digit
// K in the array elements
void sortOccurrences(int arr[],
                     int N, int K)
{
    // Stores the occurrences of K
    // in each element
    multimap<int, int> mp;

    // Traverse the array
    for (int i = 0; i < N; i++) {

        // Count the frequency
        // of K in arr[i]
        int count = countOccurrences(
            arr[i], K);

        // Insert elements in mp
        mp.insert(pair<int, int>(
            count, arr[i]));
    }

    // Print the elements in the map, mp
    for (auto& itr : mp) {
        cout << itr.second << " ";
    }
}

// Driver Code
int main()
{
    int arr[] = { 15, 66, 26, 91 };
    int K = 6;
    int N = sizeof(arr) / sizeof(arr[0]);

    // Function Call
    sortOccurrences(arr, N, K);

    return 0;
}

Java

// Java program for the above approach
import java.io.*;
import java.lang.*;
import java.util.*;

class GFG {

  // Function to count the occurrences
  // of digit K in an element
  static int countOccurrences(int num, int K)
  {
    // If num and K are both 0
    if (K == 0 && num == 0)
      return 1;

    // Initialize count as 0
    int count = 0;

    // Count for occurrences of digit K
    while (num > 0) {
      if (num % 10 == K)
        count++;
      num /= 10;
    }

    // Return the count
    return count;
  }

  // Pair class
  static class Pair {

    int idx;
    int freq;

    Pair(int idx, int freq)
    {
      this.idx = idx;
      this.freq = freq;
    }
  }

  // Function to print the given array
  // in increasing order of the digit
  // K in the array elements
  static void sortOccurrences(int arr[], int N, int K)
  {

    // Stores the Pair with index
    // and occurrences of K of each element
    Pair mp[] = new Pair[N];

    // Traverse the array
    for (int i = 0; i < N; i++) {

      // Count the frequency
      // of K in arr[i]
      int count = countOccurrences(arr[i], K);

      // Insert Pair in mp
      mp[i] = new Pair(i, count);
    }

    // sort the mp in increasing order of freq
    // if freq equal then according to index
    Arrays.sort(mp, (p1, p2) -> {
      if (p1.freq == p2.freq)
        return p1.idx - p2.idx;
      return p1.freq - p2.freq;
    });

    // Print the elements in the map, mp
    for (Pair p : mp) {
      System.out.print(arr[p.idx] + " ");
    }
  }

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

    int arr[] = { 15, 66, 26, 91 };
    int K = 6;
    int N = arr.length;

    // Function Call
    sortOccurrences(arr, N, K);
  }
}

// This code is contributed by Kingash.

Python3

# Python program for the above approach

# Function to count the occurrences
# of digit K in an element
def countOccurrences( num, K):
  
    # If num and K are both 0
    if (K == 0 and num == 0):
        return 1
      
    # Initialize count as 0
    count = 0
    
    # Count for occurrences of digit K
    while (num > 0):
        if (num % 10 == K):
            count += 1
        num //= 10
        
    # Return the count
    return count

# Function to print the given array
# in increasing order of the digit
# K in the array elements
def sortOccurrences(arr, N, K):
  
    # Stores the occurrences of K
    # in each element
    mp = []
    
    # Traverse the array
    for i in range(N):
      
        # Count the frequency
        # of K in arr[i]
        count = countOccurrences(arr[i], K)
        
        # Insert elements in mp
        mp.append([count, arr[i]])

    # Print the elements in the map, mp
    mp = sorted(mp)
    
    for itr in mp:
        print(itr[1], end = ' ')

# Driver Code
arr = [ 15, 66, 26, 91 ]
K = 6
N = len(arr)

# Function Call
sortOccurrences(arr, N, K);

# This code is contributed by rohitsingh07052.

// C# program for the above approach
using System;

public class GFG 
{

  // Function to count the occurrences
  // of digit K in an element
  static int countOccurrences(int num, int K)
  {
    // If num and K are both 0
    if (K == 0 && num == 0)
      return 1;

    // Initialize count as 0
    int count = 0;

    // Count for occurrences of digit K
    while (num > 0) {
      if (num % 10 == K)
        count++;
      num /= 10;
    }

    // Return the count
    return count;
  }

  // Pair class
  class Pair : IComparable<Pair> {

    public int idx;
    public int freq;

    public Pair(int idx, int freq)
    {
      this.idx = idx;
      this.freq = freq;
    }
      public int CompareTo(Pair p)
      {
          if (this.freq == p.freq)
            return this.idx - p.idx;
          return this.freq - p.freq;

      }
  }

  // Function to print the given array
  // in increasing order of the digit
  // K in the array elements
  static void sortOccurrences(int []arr, int N, int K)
  {

    // Stores the Pair with index
    // and occurrences of K of each element
    Pair []mp = new Pair[N];

    // Traverse the array
    for (int i = 0; i < N; i++) {

      // Count the frequency
      // of K in arr[i]
      int count = countOccurrences(arr[i], K);

      // Insert Pair in mp
      mp[i] = new Pair(i, count);
    }

    // sort the mp in increasing order of freq
    // if freq equal then according to index
    Array.Sort(mp);

    // Print the elements in the map, mp
    foreach (Pair p in mp) {
      Console.Write(arr[p.idx] + " ");
    }
  }

  // Driver Code
  public static void Main(String[] args)
  {

    int []arr = { 15, 66, 26, 91 };
    int K = 6;
    int N = arr.Length;

    // Function Call
    sortOccurrences(arr, N, K);
  }
}

// This code is contributed by 29AjayKumar

JavaScript

<script>

// JavaScript program for the above approach

// Function to count the occurrences
// of digit K in an element
function countOccurrences( num, K){
  
    // If num and K are both 0
    if (K == 0 && num == 0)
        return 1
      
    // Initialize count as 0
    count = 0
    
    // Count for occurrences of digit K
    while (num > 0){
        if (num % 10 == K)
            count += 1
        num = Math.floor(num/10)
    }
        
    // Return the count
    return count
}

// Function to print the given array
// in increasing order of the digit
// K in the array elements
function sortOccurrences(arr, N, K){
  
    // Stores the occurrences of K
    // in each element
    mp = []
    
    // Traverse the array
    for(let i=0;i<N;i++){
      
        // Count the frequency
        // of K in arr[i]
        let count = countOccurrences(arr[i], K)
        
        // Insert elements in mp
        mp.push([count, arr[i]])
    }

    // Print the elements in the map, mp
    mp.sort()
    
    for([itr1,itr2] of mp)
        document.write(itr2,' ')
}

// Driver Code
let arr = [ 15, 66, 26, 91 ]
let K = 6
let N = arr.length

// Function Call
sortOccurrences(arr, N, K)

// This code is contributed by shinjanpatra

</script>

Output:

15 91 26 66

Time Complexity: O(N*log₁₀N)
Auxiliary Space: O(N)

Analysis of Algorithms

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Sort an array according to the increasing frequency of the digit K in the array elements

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