Given an integer array nums, return all unique triplets [nums[i], nums[j], nums[k]] (i, j, k all different) that sum to 0. Each triplet's numbers should be sorted ascending, and the triplets themselves may be returned in any order — no duplicate triplets.
Sort first, fix one number, then two-point the rest — the same shape as Two Sum II, one level up.
Example
Input: nums = [-1, 0, 1, 2, -1, -4]
Output: [[-1, -1, 2], [-1, 0, 1]]