424 Longest Repeating Character Replacement

You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

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.

Constraints:

• 1 <= s.length <= 105
• s consists of only uppercase English letters.
• 0 <= k <= s.length

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class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        win_start = 0
        max_len = 0
        count_map = defaultdict(int)  # char count within the current window
        max_count = 0

        for win_end, c in enumerate(s):
            count_map[c] += 1
            max_count = max(max_count, count_map[c])

            if win_end - win_start + 1 - max_count > k:
                count_map[s[win_start]] -= 1
                win_start += 1
            
            max_len = max(max_len, win_end - win_start + 1)
        return max_len