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Permutation in String
Permutation in String is less about the final answer and more about disciplined window bookkeeping. Interviewers love it because it reveals your stateful scanning and your edge-case discipline. Be disciplined about counts and boundaries; most bugs are off-by-one window moves.
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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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