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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.

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)

代码如下：

class Solution {public:    vector
   
    
      > fourSum(vector
     
       &num, int target) {		vector
      
       
        > out;		set
        
         
          > res; if (num.size() < 4) return out; sort(num.begin(), num.end()); for (int i=0; i