Fixed sliding window

easy

Consecutive windows share k−1 elements, so recomputing from scratch is waste. Add the entering value, subtract the leaving one - O(1) per position.

Worst O(n)Space O(1)

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Where to go next

Variable sliding window

Grow right, shrink left, never move backwards.

Prefix sums

Precompute once, answer every range query in O(1).

Kadane's algorithm

One decision, repeated n times.