731. My Calendar II

Given a string that consists of only uppercase English letters, you can replace any letter in the string with another letter at most k times. Find the length of a longest substring containing all repeating letters you can get after performing the above operations.

Note:
Both the string’s length and k will not exceed 104.

Example 1:

Input:
s = "ABAB", k = 2

Output:
4

Explanation:
Replace the two 'A's with two 'B's or

Example 2:

Input:
s = "AABABBA", k = 1

Output:
4

Explanation:
Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.

class Solution {
    public int characterReplacement(String s, int k) {
        if (s.length() == 0) return 0;
        int res = 0;
        int l = 0;
        int[] letters = new int[26];

        int maj = 0;

        for (int r = 0; r < s.length(); r++) {
            char c = s.charAt(r);
            maj = Math.max(maj, ++ letters[c -'A']);
            int winSize = r - l + 1;
            if (winSize - maj <= k) {
                res = Math.max(winSize, res);
            } else {
                letters[s.charAt(l++) - 'A'] --;
            }
        }
        return