Java Sliding Window, 12 lines with explanation.


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    public class Solution {
        public int characterReplacement(String s, int k) {
            int[] map = new int[26];
            char[] sc = s.toCharArray();
            int l = 0, r= 0,maxLen = 0,result = 0;
            while (r < sc.length) {
                maxLen = Math.max(maxLen, ++map[sc[r++]-'A']);
                while (r - l - maxLen>k) map[sc[l++]-'A']--;
                result = Math.max(result, r - l);
            }
            return result;
        }
    }
    

    I use a map to store every character's frequency in the window. And calculate the max frequency as maxLen, so the other characters in the window can be replaced if r-l-maxLen<=k. Otherwise, shrink the window and update map while the window is valid again. Finally calculate and update the max valid window's length.


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