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.

此题题目如果能读懂的话，就算成功了，看了答案，大致意思是，遍历整个字符串，当到某一个字符的时候，end-start+1-包含字符个数最多的个数>k时候，把最左面的字符在数组里面出现的次数减一
并且start向右遍历，代码如下：

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

此题说白了就是，进行两次比较，一次记录出现字符数目最多的情形，另外一次记录出现的最大长度，while来处理数目大于k的情况。