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Problem

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.

Code

class Solution {
    public int characterReplacement(String s, int k) {
        int[] dict = new int[26];
        
        int start = 0;
        int maxCount = 0;
        int res = 0;
        
        for (int end = 0; end < s.length(); end++) {
            maxCount = Math.max(maxCount, ++dict[s.charAt(end) - 'A']);
            
            if (end - start + 1 - maxCount > k) {
                dict[s.charAt(start) - 'A']--;
                start++;
            }
            
            res = Math.max(res, end - start + 1);
        }
        
        return res;
    }
}