[sliding window] 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.

用sliding window,但是几点要注意。
什么时候往右扩大window?只要当前 窗口长度 不超过 最大的重复字母count + k(k可以理解为容错度)
什么时候调整window的起始?当以上条件不符时
maxcount代表什么?不是当前窗口内的 最大重复字母count,而是史上最大的重复字母count,我们只关心史上出现的最大valid串长度

public int characterReplacement(String s, int k) {
    int start = 0, maxlen = 0, maxcount = 0; 
    int[] count = new int[26];
    for (int end = 0; end < s.length(); end++) {
        maxcount = Math.max(maxcount, ++count[s.charAt(end) - 'A']);
        while (end - start + 1 > maxcount + k) {
            count[s.charAt(start++) - 'A']--;
        } 
        maxlen = Math.max(maxlen, end - start + 1);
    }
    return maxlen;
}
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