4019. Merge Close Characters II

You are given a string s consisting of lowercase English letters and an integer k.

Two equal characters s[i] and s[j], where 0 <= i < j < s.length, are considered close if j - i <= k. All indices refer to the current string.

Repeatedly perform the following operation until no close pair remains:

• Among all close pairs (i, j), choose the pair with the smallest i. If multiple pairs have the same i, choose the one with the smallest j.
• Merge the right character into the left character by removing s[j] from s. The character s[i] remains unchanged, and the remaining characters are reindexed.

Return the resulting string after performing all possible merges.

class Solution {
public:
    string mergeCharacters(string s, int k) {
        string res = "";

        for(auto& ch : s) {
            bool ok = false;

            int start = max(0, int(res.length()) - k);

            for(int j = start; j < res.length(); j++) {
                if(res[j] == ch) ok = true;
            }

            if(!ok) res.push_back(ch);
        }

        return res;
    }
};