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3Sum
3Sum is Two Sum with a twist: sort first, then walk two pointers inward for each anchor. The real trick is skipping duplicates cleanly so your answer set stays unique. Nail that and you have a template for 4Sum and every k-sum that follows.
Learn this pattern
Arrays and Hashing Interview Questions
Arrays and hashing problems are about fast lookup choices: set for membership, map for attached state, and buckets when counts are bounded. Reach for this family when brute force compares too many pairs or when you need to remember what you saw earlier in one pass.
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