Longest subsequence where each character occurs at least k times

Given a string 's' and an integer k, find other string 't' such that 't' is the largest subsequence of given string 's' and each character of 't' must occur at least k times in string s.

Examples :

Input s = "geeksforgeeks" k = 2
Output geeksgeeks
Explanation 'g', 'e', 'k', and 's' appear twice or more, so the output is "geeksgeeks"

Input s = "baaabaacba" k = 3
Output baaabaaba
Explanation Characters 'b' and 'a' appear at least 3 times, so the result is "baaabaaba".

[Naive Approach] Generate All Subsequences - Exponential time

We can solve the problem by generating all subsequences of the string. For each subsequence, we check if every character appears at least k times. Among all valid subsequences, we keep track of the longest one.

[Efficient Approach] Using Frequency Counting - O(n) time and O(1) space

• We use a counter array to store the frequency of each character in the string.
• Then, iterate through the string, and for each character, check if its count is greater than or equal to k. If it is, include that character in the result.

geeksgeeks