4Sum

Given an array `nums` of `n` integers, return _an array of all the **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**.   **Example 1:** **Input:** nums = [1,0,-1,0,-2,2], target = 0 **Output:** [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]] ``` **Example 2:** **Input:** nums = [2,2,2,2,2], target = 8 **Output:** [[2,2,2,2]] ```   **Constraints:** - `1 <= nums.length <= 200` - `-109 <= nums[i] <= 109` - `-109 <= target <= 109`