https://leetcode.com/problems/longest-repeating-character-replacement/

// sliding window

public class Solution {

    public int characterReplacement(String s, int k) {

        int[] count = new int[26];

        int maxch = -1; // the char with max count in current substr

        int max = 0; // the max count for single char in current substr

        int len = 0; // current substr length

        int ret = 0; // final result

        for (int i=0; i<s.length(); i++) {

            int tmp = s.charAt(i) - 'A';

            count[tmp]++;

            len++;

            if (maxch == tmp) {

                max++;

            }

            else {

                if (count[tmp] > max) {

                    max = count[tmp];

                    maxch = tmp;

                }

            }

            if (len - max <= k) {

                if (len > ret) {

                    ret = len;

                }

            }

            while (len - max > k) {

                int newTmp = s.charAt(i-len+1) - 'A';

                count[newTmp]--;

                len--;

                if (maxch == newTmp) {

                    max--;

                    for (int j=0; j<26; j++) {

                        if (count[j] > max) {

                            max = count[j];

                            maxch = j;

                        }

                    }

                }

            }

        }

        return ret;

    }

}