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# LeetCode problem 424 Longest Repeating Character Replacement(Java)

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.

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.