LeetCode 424. Longest Repeating Character Replacement

class Solution {
public:
    int characterReplacement(string s, int k) {
        int n = s.size(), left = 0, right = 0, maxCnt = 0;
        vector<int> freq(26);
        while (right < n) {
            maxCnt = max(maxCnt, ++freq[s[right++] - 'A']);
            if (right - left > maxCnt + k) {
                --freq[s[left++] - 'A'];
            }
        }
        return right - left;
    }
};