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:

    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
      
        tmp; tmp.push_back(num[i]); tmp.push_back(num[j]); tmp.push_back(num[begin]); tmp.push_back(num[end]); res.insert(tmp); begin++; end--; } else if(sum < target) { begin++; }else{ end--; } } } } set
       
        >::iterator it = res.begin(); for(; it != res.end(); it++) out.push_back(*it); } };