Modify string by sorting characters after removal of characters whose frequency is not equal to power of 2

Given a string S consisting of N lowercase alphabets, the task is to remove the characters from the string whose frequency is not a power of 2 and then sort the string in ascending order.

Examples:

Input: S = “aaacbb”
Output: bbc
Explanation: The frequencies of ‘a’, ‘b’, and ‘c’ in the given string are 3, 2, 1. Only the character ‘a’ has frequency (= 3) which is not a power of 2. Therefore, removing ‘a’ from the string S modifies it to “cbb”. Therefore, the modified string after sorting is “bbc”.

Input: S = “geeksforgeeks”
Output: eeeefggkkorss

Approach: The given problem can be solved by using Hashing. Follow the steps below to solve the given problem:

• Initialize an auxiliary array of size 26, say freq[], to store the frequency of each character in the string S such that index 0 represents character ‘a’, 1 represents the character ‘b’, and so on.
• Traverse the given string S and increment the frequency of each character S[i] by 1 in the array freq[].
• Traverse the array freq[] and if the value of freq[i] is a power of 2, then print the character (‘a’ + i), freq[i] number of times.

Below is the implementation of the above approach:

C++

// C++ program for the above approach

#include <bits/stdc++.h>
using namespace std;

// Function to remove all the characters
// from a string that whose frequencies
// are not equal to a perfect power of 2
void countFrequency(string S, int N)
{
    // Stores the frequency of
    // each character in S
    int freq[26] = { 0 };

    // Iterate over characters of string
    for (int i = 0; i < N; i++) {

        // Update frequency of current
        // character in the array freq[]
        freq[S[i] - 'a']++;
    }

    // Traverse the array freq[]
    for (int i = 0; i < 26; i++) {

        // Check if the i-th letter
        // is absent from string S
        if (freq[i] == 0)
            continue;

        // Calculate log of frequency
        // of the current character
        // in the string S
        int lg = log2(freq[i]);

        // Calculate power of 2 of lg
        int a = pow(2, lg);

        // Check if freq[i]
        // is a power of 2
        if (a == freq[i]) {

            // Print letter i + 'a'
            // freq[i] times
            while (freq[i]--)
                cout << (char)(i + 'a');
        }
    }
}

// Driver Code
int main()
{
    string S = "aaacbb";
    int N = S.size();
    countFrequency(S, N);

    return 0;
}

Java

// Java program for the above approach
class GFG{

// Function to remove all the characters
// from a string that whose frequencies
// are not equal to a perfect power of 2
static void countFrequency(String S, int N)
{

    // Stores the frequency of
    // each character in S
    int []freq = new int[26];
    //Array.Clear(freq, 0, freq.Length);

    // Iterate over characters of string
    for(int i = 0; i < N; i++)
    {

        // Update frequency of current
        // character in the array freq[]
        freq[(int)S.charAt(i) - 'a'] += 1;
    }

    // Traverse the array freq[]
    for(int i = 0; i < 26; i++)
    {

        // Check if the i-th letter
        // is absent from string S
        if (freq[i] == 0)
            continue;

        // Calculate log of frequency
        // of the current character
        // in the string S
        int lg = (int)(Math.log(freq[i]) / Math.log(2));

        // Calculate power of 2 of lg
        int a = (int)Math.pow(2, lg);

        // Check if freq[i]
        // is a power of 2
        if (a == freq[i])
        {

            // Print letter i + 'a'
            // freq[i] times
            while (freq[i] > 0)
            {
                freq[i] -= 1;
                System.out.print((char)(i + 'a'));
            }
        }
    }
}

// Driver Code
public static void main(String args[])
{
    String S = "aaacbb";
    int N = S.length();

    countFrequency(S, N);
}
}

// This code is contributed by AnkThon

Python3

# Python3 program for the above approach
from math import log2

# Function to remove all the characters
# from a that whose frequencies are not
# equal to a perfect power of 2
def countFrequency(S, N):

    # Stores the frequency of
    # each character in S
    freq = [0] * 26

    # Iterate over characters of string
    for i in range(N):

        # Update frequency of current
        # character in the array freq[]
        freq[ord(S[i]) - ord('a')] += 1

    # Traverse the array freq[]
    for i in range(26):

        # Check if the i-th letter
        # is absent from S
        if (freq[i] == 0):
            continue

        # Calculate log of frequency
        # of the current character
        # in the S
        lg = int(log2(freq[i]))

        # Calculate power of 2 of lg
        a = pow(2, lg)

        # Check if freq[i]
        # is a power of 2
        if (a == freq[i]):

            # Print letter i + 'a'
            # freq[i] times
            while (freq[i]):
                print(chr(i + ord('a')), end = "")
                freq[i] -= 1

# Driver Code
if __name__ == '__main__':

    S = "aaacbb"
    N = len(S)

    countFrequency(S, N)

# This code is contributed by mohit kumar 29

C#

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

class GFG{

// Function to remove all the characters
// from a string that whose frequencies
// are not equal to a perfect power of 2
static void countFrequency(string S, int N)
{

    // Stores the frequency of
    // each character in S
    int []freq = new int[26];
    Array.Clear(freq, 0, freq.Length);

    // Iterate over characters of string
    for(int i = 0; i < N; i++)
    {

        // Update frequency of current
        // character in the array freq[]
        freq[(int)S[i] - 'a'] += 1;
    }

    // Traverse the array freq[]
    for(int i = 0; i < 26; i++)
    {

        // Check if the i-th letter
        // is absent from string S
        if (freq[i] == 0)
            continue;

        // Calculate log of frequency
        // of the current character
        // in the string S
        int lg = (int)Math.Log((double)freq[i], 2.0);

        // Calculate power of 2 of lg
        int a = (int)Math.Pow(2, lg);

        // Check if freq[i]
        // is a power of 2
        if (a == freq[i])
        {

            // Print letter i + 'a'
            // freq[i] times
            while (freq[i] > 0)
            {
                freq[i] -= 1;
                Console.Write((char)(i + 'a'));
            }
        }
    }
}

// Driver Code
public static void Main()
{
    string S = "aaacbb";
    int N = S.Length;

    countFrequency(S, N);
}
}

// This code is contributed by SURENDRA_GANGWAR

Javascript

<script>
      // JavaScript program for the above approach
      // Function to remove all the characters
      // from a string that whose frequencies
      // are not equal to a perfect power of 2
      function countFrequency(S, N)
      {

        // Stores the frequency of
        // each character in S
        var freq = new Array(26).fill(0);

        // Iterate over characters of string
        for (var i = 0; i < N; i++)
        {

          // Update frequency of current
          // character in the array freq[]
          freq[S[i].charCodeAt(0) - "a".charCodeAt(0)]++;
        }

        // Traverse the array freq[]
        for (var i = 0; i < 26; i++)
        {

          // Check if the i-th letter
          // is absent from string S
          if (freq[i] === 0) continue;

          // Calculate log of frequency
          // of the current character
          // in the string S
          var lg = parseInt(Math.log2(freq[i]));

          // Calculate power of 2 of lg
          var a = Math.pow(2, lg);

          // Check if freq[i]
          // is a power of 2
          if (a === freq[i])
          {

            // Print letter i + 'a'
            // freq[i] times
            while (freq[i]--)
              document.write(String.fromCharCode(i + "a".charCodeAt(0)));
          }
        }
      }

      // Driver Code
      var S = "aaacbb";
      var N = S.length;
      countFrequency(S, N);

      // This code is contributed by rdtank.
    </script>

Output:

bbc

Time Complexity: O(N)
Auxiliary Space: O(1)