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Longest Repeating Character Replacement
The window is valid while its length minus its most frequent count stays within k. I grow, then shrink only when that breaks.
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Sliding Window Algorithm
Sliding window processes arrays or strings by moving a window through the data, updating results incrementally instead of recalculating from scratch. Reach for it when you see "contiguous subarray," "longest/shortest substring," or "at most K distinct."
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