Medium
Sliding Window
Longest Repeating Character Replacement
A window is valid while (length − count of its most frequent char) ≤ k.
Approach
Slide a window tracking character counts and the max single-character count inside it. If the window's length minus that max exceeds k, too many replacements are needed — shrink from the left. The widest valid window is the answer.
Time complexity
O(n)
Space complexity
O(1)
Common mistake
Recomputing the true max frequency on every shrink; you can let a slightly stale max stand because the window only ever needs to grow past its best.
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