424. Longest Repeating Character Replacement 改变k个字母，形成最长的连续

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.

Example 1:

Input: s = "ABAB", k = 2
Output: 4
Explanation: Replace the two 'A's with two 'B's or vice versa.

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.

滑动窗口可以不限制多大，只限制左右
aaababb:所以不能只看最长啊，计算长度的方法是int lettersToChange = (right - left + 1) - mostFreqLetter;
参考：https://leetcode.com/problems/longest-repeating-character-replacement/discuss/358879/Java-Solution-Explained-and-Easy-to-Understand-for-Interviews

class Solution {
    public int characterReplacement(String s, int k) {
        int[] freq = new int[26];
        int mostFreqLetter = 0;
        int left = 0;
        int max = 0;
        
        for(int right = 0; right < s.length(); right++){
            freq[s.charAt(right) - 'A']++;
            mostFreqLetter = Math.max(mostFreqLetter, freq[s.charAt(right) - 'A']);
            
            int lettersToChange = (right - left + 1) - mostFreqLetter;
            if(lettersToChange > k){
                freq[s.charAt(left) - 'A']--;
                left++;
            }
            
            max = Math.max(max, right - left + 1);
        }
        
        return max;
    }
}