Given a string s and an integer k, return the length of the longest substring that can be turned into one repeated character by changing at most k of its characters. A window of length L is achievable with at most k replacements exactly when L - maxFreq ≤ k, where maxFreq is the count of the window's most frequent character — every other character in the window is the one that would need replacing. The sliding windowSliding WindowMaintaining a running result over a contiguous range that grows or shrinks one element at a time, instead of recomputing the result for every range from scratch. technique grows the window right while tracking a frequency count and the running maxFreq, and shrinks from the left by one whenever that condition breaks. maxFreq is allowed to go stale after a shrink — it never causes an invalid window to be accepted, since the tracked answer only advances when a genuinely longer valid window is found — so the whole scan still runs in O(n).