18. 4Sum

Given an array nums of n integers, return an array of all unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that: • 0 <= a, b, c, d < n • a, b, c, and d are distinct. • nums[a] + nums[b] + nums[c] + nums[d] == target You may return the answer in any order.

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