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 = 2Output:4Explanation:Replace the two 'A's with two 'B's or vice versa.

Example 2:

Input:s = "AABABBA", k = 1Output:4Explanation:Replace the one 'A' in the middle with 'B' and form "AABBBBA".The substring "BBBB" has the longest repeating letters, which is 4.

滑动窗口，考虑当前窗口里的字符，除了出现最多次数的字符之外还有多少个其他字符，如果置换这些其他字符需要的次数大于K，则需要缩短窗口（缩短左边界），如果置换其他字符需要的次数<=K，则表明窗口里是一种合适的置换，记录当前长度

public class Solution {    public int characterReplacement(String s, int k){int len=s.length();int[] carr=new int[128];int start=0,end=0;int maxCount=0;int maxLen=0;for(end=0;end<len;end++){char c=s.charAt(end);maxCount=Math.max(maxCount, ++carr[c]);while(end-start+1-maxCount>k){carr[s.charAt(start)]--;start++;}maxLen=Math.max(maxLen, end-start+1);}return maxLen;}}