leetcode 4Sum 四数之和

Given an array S of n integers, are there elements a, b, c, and d in S 
such that a + b + c + d = 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.
    For 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)
solution: