4Sum

Given an array S of n integers, are there elements a, b, c, and d in S
such that a + b + c + d = target? Find all unique quadruplets in the array
which gives the sum of target.
Note
Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a <= b <= c <= d)
The solution set must not contain duplicate quadruplets.
Example
Given array S = {1 0 -1 0 -2 2}, and target = 0.
A solution set is:
(-1, 0, 0, 1)
(-2, -1, 1, 2)
(-2, 0, 0, 2)

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