Longest Repeating Character Replacement

lcpy gen -n 424   # by problem number
lcpy gen -s longest_repeating_character_replacement   # by problem name

Input: s = "ABAB", k = 2
Output: 4
Explanation: Replace the two 'A's with two 'B's or vice versa.

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.
There may exists other ways to achieve this answer too.

class Solution:
    # Time: O(n) - single pass through string
    # Space: O(1) - at most 26 characters in count dict
    def character_replacement(self, s: str, k: int) -> int:
        """
        Find the length of the longest substring with same character
        after at most k replacements using sliding window approach.
        """
        if not s:
            return 0

        count: dict[str, int] = {}
        left = 0
        max_freq = 0
        max_length = 0

        for right in range(len(s)):
            # Expand window: add character at right pointer
            count[s[right]] = count.get(s[right], 0) + 1
            max_freq = max(max_freq, count[s[right]])

            # Shrink window if needed: if we need more than k replacements
            # Current window size = right - left + 1
            # Characters to replace = window_size - max_freq
            # If characters_to_replace > k, we need to shrink
            if (right - left + 1) - max_freq > k:
                count[s[left]] -= 1
                left += 1

            # Update max length
            max_length = max(max_length, right - left + 1)

        return max_length