Sliding Window

A moving range that grows and shrinks in O(n).

sliding_window

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Window "a" — longest so far: "a" (1).

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How it works

The sliding-window pattern maintains a contiguous range that expands and contracts as it scans, reusing work instead of recomputing each subarray. It turns brute-force O(n·k) substring and subarray problems into a single O(n) sweep.

Mental models

Expand the right edge to include; shrink the left to stay valid.
Each element enters and leaves the window at most once → O(n).

Complexity

Time: O(n)
Space: O(k)window contents

Reach for it when

Longest substring problems
Max sum subarray of size k
Rate limiting