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Two pointers, one pass, O(n) — learn how the variable-size sliding window grows and shrinks to solve substring and subarray problems efficiently.

Stop re-summing from scratch — the fixed-size sliding window reduces O(n·k) nested loops to a single O(n) pass by adding one element and dropping one.

Most people reach for the nested loop on Two Sum — here's why the hash map pattern is the interview move, plus 3Sum and sorted variants.

LeetCode 4