Two Sum & Beyond — 21 Solved Problems (Java)

A solved problem set built around the Two Sum family and the patterns it teaches — two pointers, prefix sums with hashing, and pair-counting — across 21 LeetCode problems from easy to hard, including 3Sum, 4Sum, Subarray Sum Equals K, Max Number of K-Sum Pairs, and Number of Excellent Pairs. Every problem gets a plain-English approach, working Java solution code, and its time/space complexity, with high-frequency interview patterns flagged so you know what to master first.