Find longest uniform substring after k replacements

Given a string s (ASCII, length up to 2e5) and integer k (0 ≤ k ≤ |s|), return the length of the longest substring that can be turned into all the same character by replacing at most k characters. Constraints and requirements: - Time O(n), space O(1) amortized (treat alphabet size as constant). - Explain why a sliding window with a running max‑frequency works and why shrinking the window preserves correctness even as the max frequency may lag. - Provide examples: s="AABABBA", k=1 ⇒ 4; s="aaabbc", k=2 ⇒ 5; s="abcd", k=1 ⇒ 2. - Implement in your preferred language and discuss edge cases (k=0, all identical chars, very large n).