Given a string s and an integer k, you can replace at most k characters with any other character. Return the length of the longest substring containing the same letter after performing at most k replacements.
With s = "AABABBA" and k = 1, the answer is 4. You can replace one B to get "AAAA" (a substring of the modified string).
Think about it: in any valid window, if you know the most frequent character, how many replacements do you need to make all characters match?
Constraints: , .