Solve Two Sum 3 ways: (1) Brute force O(n²), (2) Hash map O(n) time O(n) space, (3) Two pointers after sorting O(n log n) time O(1) space. Then solve variants: (a) Three Sum (find all unique triplets summing to 0), (b) Two Sum II (sorted input — optimize to O(n)), (c) Two Sum less than K (count pairs with sum < K). Explain tradeoffs and when to use each approach.