424. Longest Repeating Character Replacement

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 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.

　　窗口指针状态的更新和题意的结合: 

count = Math.max(count, ++sum[s.charAt(fast) - 'A']);
            while (fast - slow + 1 - count > k) {
                sum[s.charAt(slow) - 'A']--;
                slow++;
            }

public int characterReplacement(String s, int k) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        int[] sum = new int[26];
        int slow = 0, fast = 0, count = 1, ans = 1;
        while (fast < s.length()) {
            count = Math.max(count, ++sum[s.charAt(fast) - 'A']);
            while (fast - slow + 1 - count > k) {
                sum[s.charAt(slow) - 'A']--;
                slow++;
            }
            
            ans = Math.max(ans, fast - slow + 1);
            fast++;
        }
        return ans;
    }