424. Longest Repeating Character Replacement

You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

class Solution {
public:
    int characterReplacement(string s, int k) {
        unordered_map<char, int> counter;
        int res = 0, l = 0, r = 0, ma = 0, n = s.length();
        while(r < n) {
            while(r < n and r - l - ma <= k) {
                res = max(res, r - l);
                ma = max(ma, ++counter[s[r++]]);
            }
            
            while(l < r and r - l - ma > k) {
                --counter[s[l++]];
            }
        }
        res = max(res, r - l);
        return res;
    }
};