424. Longest Repeating Character Replacement

This question asks us to find the longest window within a string where we can replace up to k characters. This is a sliding window question, where we expand the window if we have at least one extra character or we add to the most common character. So, we do that.

class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        l = 0
        max_len = 0
        freq = defaultdict(int)
        for r, c in enumerate(s):
            freq[c] += 1
             
            curr = r - l + 1
            if curr - max(freq.values()) <= k:  
                max_len = max(max_len, curr)
            else:                               
                freq[s[l]] -= 1                 
                l += 1
               
        return max_len