Longest Repeating Character Replacement in C#

You are given a string s made of uppercase English letters and an integer k. You can pick any character of s and change it into any other uppercase letter. You can do this at most k times.

Return the length of the longest substring made of one repeated letter that you can get after these changes.

A substring is a run of consecutive characters of s.

Example 1:

Input: s = "ABAB", k = 2
Output: 4
Explanation: Change both A's into B's to get "BBBB", or both B's into A's to get "AAAA".

Example 2:

Input: s = "AABABBA", k = 1
Output: 4
Explanation: Change the A at index 3 into a B to get "AABBBBA": the substring "BBBB" has length 4. Changing the B at index 2 gives "AAAA" instead. No single change gives a run of 5.

Example 3:

Input: s = "ABBBAABBBBA", k = 2
Output: 9
Explanation: Change the A's at indexes 4 and 5 into B's: indexes 1 to 9 become "BBBBBBBBB". Any longer substring holds a third A.

Constraints:

• 1 <= s.length <= 105
• s is made of uppercase English letters only (A to Z).
• 0 <= k <= s.length