Source Url：https://leetcode.com/problems/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 10^{4}.

**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.

This problem is a standard resizable sliding window problem.

Every time when a character moves into the window, we need to count how many characters we can replace. We need to replace the inference character. For example “ABCCD”, if we can replace 3 characters, we must keep “CC” unchanged, replace “ABD” as “CCC”. When replaced characters are more than the value of k, we need to move left to let a character out of the window.

So function ** characterReplacement** is very familiar (see also).

Then is function ** replaceCount** to count we need to replace how many characters to make sure all characters is the same.

Running time is 30ms.

Code download address: https://github.com/tinyfool/leetcode/tree/master/src/p0424

Want to know details of the sliding window algorithm, check my post: The Sliding Window Algorithm for string and array.