Implement:
three_sum(nums): all unique triplets [a, b, c] with a + b + c == 0. Each triplet is sorted, and the list of triplets is sorted. Run in O(n²) by sorting first and using two pointers, skipping duplicates.three_sum_closest(nums, target): the sum of three elements closest to target (if two sums are equally close, return the smaller one)