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# Find the length of the longest subsequence with first K alphabets having same frequency

• Last Updated : 28 May, 2021

Given string str with uppercase characters and an integer K, the task is to find the length of the longest subsequence such that the frequency of the first K alphabet is the same.

Examples:

Input: str = “ACAABCCAB”, K=3
Output:
Explanation: One of the possible subsequences is “ACABCB”.

Input: str = “ACAABCCAB”, K=4
Output:
Explanation: Since the string does not contain ‘D’, no such subsequence can be obtained.

Approach:
Traverse the string and find the least frequent of the first K alphabets. Once found, (frequency of that element) * K gives the desired result.
Below is the implementation of the above approach:

## C++

 `// C++ program to find the longest``// subsequence with first K``// alphabets having same frequency` `#include ``using` `namespace` `std;` `// Function to return the``// length of the longest``// subsequence with first K``// alphabets having same frequency``int` `lengthOfSubsequence(string str,``                            ``int` `K)``{``    ``// Map to store frequency``    ``// of all characters in``    ``// the string``    ``map<``char``,``int``> mp;``    ``for` `(``char` `ch : str) {``        ``mp[ch]++;``    ``}``    ` `    ``// Variable to store the``    ``// frequency of the least``    ``// frequent of first K``    ``// alphabets``    ``int` `minimum = mp[``'A'``];``    ``for` `(``int` `i = 1; i < K; i++) {``        ``minimum = min(minimum,``                    ``mp[(``char``)(i + ``'A'``)]);``    ``}``    ` `    ``return` `minimum * K;``}` `int` `main()``{``    ``string str = ``"ACAABCCAB"``;``    ``int` `K = 3;``    ` `    ``cout << lengthOfSubsequence(str, K);``    ``return` `0;``}`

## Java

 `// Java program to find the longest``// subsequence with first K alphabets``// having same frequency``import` `java.util.*;` `class` `GFG{` `// Function to return the``// length of the longest``// subsequence with first K``// alphabets having same frequency``static` `int` `lengthOfSubsequence(String str,``                            ``int` `K)``{``    ` `    ``// Map to store frequency``    ``// of all characters in``    ``// the string``    ``Map mp = ``new` `HashMap<>();``    ``for``(``char` `ch : str.toCharArray())``    ``{``        ``mp.put(ch, mp.getOrDefault(ch, ``0``) + ``1``);``    ``}` `    ``// Variable to store the``    ``// frequency of the least``    ``// frequent of first K``    ``// alphabets``    ``int` `minimum = mp.get(``'A'``);``    ``for``(``int` `i = ``1``; i < K; i++)``    ``{``        ``minimum = Math.min(minimum,``                        ``mp.get((``char``)(i + ``'A'``)));``    ``}``    ``return` `minimum * K;``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``String str = ``"ACAABCCAB"``;``    ``int` `K = ``3``;``    ` `    ``System.out.println(lengthOfSubsequence(str, K));``}``}` `// This code is contributed by offbeat`

## Python3

 `# Python3 program to find the longest``# subsequence with first K alphabets``# having same frequency``from` `collections ``import` `defaultdict` `# Function to return the``# length of the longest``# subsequence with first K``# alphabets having same frequency``def` `lengthOfSubsequence(st, K):` `    ``# Map to store frequency``    ``# of all characters in``    ``# the string``    ``mp ``=` `defaultdict(``int``)``    ``for` `ch ``in` `st:``        ``mp[ch] ``+``=` `1``    ` `    ``# Variable to store the``    ``# frequency of the least``    ``# frequent of first K``    ``# alphabets``    ``minimum ``=` `mp[``'A'``]``    ` `    ``for` `i ``in` `range``(``1``, K):``        ``minimum ``=` `min``(minimum,``                    ``mp[``chr``(i ``+` `ord``(``'A'``))])``    ` `    ``return` `(minimum ``*` `K)` `# Driver code``if` `__name__ ``=``=` `"__main__"``:``    ` `    ``st ``=` `"ACAABCCAB"``    ``K ``=` `3``    ` `    ``print``(lengthOfSubsequence(st, K))` `# This code is contributed by chitranayal`

## C#

 `// C# program to find the longest``// subsequence with first K alphabets``// having same frequency``using` `System;``using` `System.Collections.Generic;` `class` `GFG{` `// Function to return the``// length of the longest``// subsequence with first K``// alphabets having same frequency``static` `int` `lengthOfSubsequence(``string` `str,``                               ``int` `K)``{``    ` `    ``// Store frequency``    ``// of all characters in``    ``// the string``    ``Dictionary<``char``,``               ``int``> mp = ``new` `Dictionary<``char``,``                                        ``int``>();``    ` `    ``foreach``(``char` `ch ``in` `str.ToCharArray())``    ``{``        ``mp[ch] = mp.GetValueOrDefault(ch, 0) + 1;``    ``}` `    ``// Variable to store the frequency``    ``// of the least frequent of first K``    ``// alphabets``    ``int` `minimum = mp[``'A'``];``    ``for``(``int` `i = 1; i < K; i++)``    ``{``        ``minimum = Math.Min(minimum,``                           ``mp[(``char``)(i + ``'A'``)]);``    ``}``    ``return` `minimum * K;``}` `// Driver code``public` `static` `void` `Main(``string``[] args)``{``    ``string` `str = ``"ACAABCCAB"``;``    ``int` `K = 3;``    ` `    ``Console.Write(lengthOfSubsequence(str, K));``}``}` `// This code is contributed by rutvik_56`

## Javascript

 ``
Output:
`6`

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