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

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: 6 
Explanation: One of the possible subsequences is “ACABCB”.

Input: str = “ACAABCCAB”, K=4 
Output: 0 
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 <bits/stdc++.h>
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<Character, Integer> 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

<script>

// Javascript program to find the longest
// subsequence with first K
// alphabets having same frequency

// Function to return the
// length of the longest
// subsequence with first K
// alphabets having same frequency
function lengthOfSubsequence(str, K)
{
    // Map to store frequency
    // of all characters in
    // the string
    var mp = new Map();

    str.split('').forEach(ch => {
        if(mp.has(ch))
            mp.set(ch, mp.get(ch)+1)
        else
            mp.set(ch, 1)
    });

    // Variable to store the
    // frequency of the least
    // frequent of first K
    // alphabets
    var minimum = mp.get('A');
    for (var i = 1; i < K; i++) {
        minimum = Math.min(minimum,
                    mp.get(String.fromCharCode(i + 'A'.charCodeAt(0))));
    }

    return minimum * K;
}

var str = "ACAABCCAB";
var K = 3;

document.write( lengthOfSubsequence(str, K));



</script>

Output:

6