3Sum

Write `three_sum(nums)` returning every distinct triplet that sums to zero. Each triplet must be sorted ascending, and no triplet may appear twice. Order of the triplets themselves does not matter. three_sum([-1, 0, 1, 2, -1, -4]) -> [[-1, -1, 2], [-1, 0, 1]] Sort, then for each index run the two-pointer scan from Two Sum II across the rest. The entire difficulty is de-duplication: skip a repeated outer value, and after recording a hit, skip repeated values under both inner pointers. Nearly every failed attempt at this problem fails there.