Remove characters that appear more than k times

23 Jun, 2022
Given a string of lowercase letters, reduce it by removing the characters which appear more than k times in the string.Examples: 
Input : str = "geeksforgeeks"
        k = 2
Output : for

Input : str = "geeksforgeeks"
        k = 3
Output : gksforgks
Approach :  
Create a hash table of 26 indexes, where 0th index represents ‘a’ and 1th index represents ‘b’ and so on. Initialize the hash table to zero.
Iterate through the string and count increment the frequency of the str[i] character in the hash table.
Now once again traverse through the string and append those characters in the new string whose frequency in the hash table is less than k and skip those which appear more than equal to k.
Time Complexity – O(N)Below is the implementation of above approach: 
C++
Java
Python3
C#
PHP
Javascript
// C++ program to reduce the string by// removing the characters which// appears more than k times#include <bits/stdc++.h>using namespace std; const int MAX_CHAR = 26; void removeChars(char arr[], int k){    // Hash table initialised to 0    int hash[MAX_CHAR] = { 0 };     // Increment the frequency of the character    int n = strlen(arr);    for (int i = 0; i < n; ++i)        hash[arr[i] - 'a']++;        // Next index in reduced string    int index = 0;    for (int i = 0; i < n; ++i) {         // Append the characters which        // appears less than k times        if (hash[arr[i] - 'a'] < k) {           arr[index++] = arr[i];        }    }     arr[index] = '\0';} int main(){    char str[] = "geeksforgeeks";    int k = 2;    removeChars(str, k);    cout << str;    return 0;}
// Java program to reduce the string by// removing the characters which// appears more than k timesimport java.util.*;class Solution{  static final int MAX_CHAR = 26;  static void removeChars(char arr[], int k){    // Hash table initialised to 0    int hash[]=new int[MAX_CHAR];         for (int i = 0; i <MAX_CHAR; ++i)    hash[i]=0;      // Increment the frequency of the character    int n = (arr).length;    for (int i = 0; i < n; ++i)        hash[arr[i] - 'a']++;         // Next index in reduced string    int index = 0;    for (int i = 0; i < n; ++i) {          // Append the characters which        // appears less than k times        if (hash[arr[i] - 'a'] < k) {           arr[index++] = arr[i];        }    }    for (int i = index; i < n; ++i)    arr[i] = ' ';}  public static void main(String args[]){    char str[] = "geeksforgeeks".toCharArray();;    int k = 2;    removeChars(str, k);    System.out.println(String.valueOf( str));} }//contributed by Arnab Kundu
# Python 3 program to reduce the string by# removing the characters which# appears more than k timesMAX_CHAR = 26 def removeChars(arr, k):         # Hash table initialised to 0    hash = [0 for i in range(MAX_CHAR)]     # Increment the frequency of the character    n = len(arr)    for i in range(n):        hash[ord(arr[i]) - ord('a')] += 1     # Next index in reduced string    index = 0    for i in range(n):                 # Append the characters which        # appears less than k times        if (hash[ord(arr[i]) - ord('a')] < k):            arr[index] = arr[i]            index += 1             arr[index] = ' '         for i in range(index):        print(arr[i], end = '') # Driver codeif __name__ == '__main__':    str = "geeksforgeeks"    str = list(str)    k = 2    removeChars(str, k) # This code is contributed by# Shashank_Sharma
// C# program to reduce the string by// removing the characters which// appears more than k timesusing System;public class Solution{     static readonly int MAX_CHAR = 26;     static void removeChars(char []arr, int k)    {        // Hash table initialised to 0        int []hash=new int[MAX_CHAR];         for (int i = 0; i <MAX_CHAR; ++i)        hash[i]=0;         // Increment the frequency of the character        int n = (arr).Length;        for (int i = 0; i < n; ++i)            hash[arr[i] - 'a']++;             // Next index in reduced string        int index = 0;        for (int i = 0; i < n; ++i) {             // Append the characters which            // appears less than k times            if (hash[arr[i] - 'a'] < k) {            arr[index++] = arr[i];            }        }        for (int i = index; i < n; ++i)        arr[i] = ' ';    }     public static void Main()    {        char []str = "geeksforgeeks".ToCharArray();;        int k = 2;        removeChars(str, k);        Console.Write(String.Join("",str));    } }// This code is contributed by PrinciRaj1992
<?php// PHP program to reduce the string by// removing the characters which// appears more than k times$MAX_CHAR = 26; function removeChars($arr, $k){    global $MAX_CHAR;         // Hash table initialised to 0    $hash = array_fill(0, $MAX_CHAR, NULL);     // Increment the frequency of    // the character    $n = strlen($arr);    for ($i = 0; $i < $n; ++$i)        $hash[ord($arr[$i]) - ord('a')]++;     // Next index in reduced string    $index = 0;    for ($i = 0; $i < $n; ++$i)    {         // Append the characters which        // appears less than k times        if ($hash[ord($arr[$i]) - ord('a')] < $k)        {            $arr[$index++] = $arr[$i];        }    }         $arr[$index] = '';    for($i = 0; $i < $index; $i++)        echo $arr[$i];} // Driver Code$str = "geeksforgeeks";$k = 2;removeChars($str, $k); // This code is contributed by ita_c?>
<script> // JavaScript program to reduce the string by// removing the characters which// appears less than k times         let  MAX_CHAR = 26;         // Function to reduce the string by// removing the characters which// appears less than k times    function removeChars(str,k)    {        // Hash table initialised to 0        let hash = new Array(MAX_CHAR);         for(let i=0;i<hash.length;i++)        {            hash[i]=0;        }        // Increment the frequency of the character        let n = str.length;        for (let i = 0; i < n; ++i) {            hash[str[i].charCodeAt(0) -            'a'.charCodeAt(0)]++;        }          // create a new empty string        let index = "";        for (let i = 0; i < n; ++i) {              // Append the characters which            // appears more than equal to k times            if (hash[str[i].charCodeAt(0) -            'a'.charCodeAt(0)] < k)            {                index += str[i];            }        }          return index;    }         // Driver Code    let str = "geeksforgeeks";    let k = 2;    document.write(removeChars(str, k));      // This code is contributed by rag2127 </script>
for
 
Time Complexity: O(n), where n represents the size of the given array.
Auxiliary Space: O(26), no extra space is required, so it is a constant.
We will scan the string and count the occurrence of all characters using built-in Counter() function.
Now once again traverse through the string and append those characters in the new string whose frequency in the frequency dictionary is less than k and skip those which appear more than equal to k.
Note: This method is applicable for all types of characters.
Python3
# Python 3 program to reduce the string# by removing the characters which# appears more than k timesfrom collections import Counter # Function to reduce the string by# removing the characters which# appears more than k timesdef removeChars(str, k):       # Using Counter function to    # count frequencies    freq = Counter(str)         # create a new empty string    res = ""     for i in range(len(str)):         # Append the characters which        # appears less than equal to k times        if (freq[str[i]] < k):            res += str[i]     return res  # Driver Codeif __name__ == "__main__":     str = "geeksforgeeks"    k = 2     print(removeChars(str, k)) # This code is contributed by vikkycirus
for
Time Complexity: O(n), where n represents the size of the given array.
Auxiliary Space: O(26), no extra space is required, so it is a constant.
andrew1234
princiraj1992
Shashank_Sharma
ukasp
vikkycirus
rag2127
tamanna17122007
frequency-counting
Technical Scripter 2018
Hash
Strings
Technical Scripter
Hash
Strings
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