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.
Both the string's length and k will not exceed 104.
s = "ABAB", k = 2
Replace the two 'A's with two 'B's or vice versa.
s = "AABABBA", k = 1
Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.