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)

class Solution {public:    vector
     
      
       > fourSum(vector
       
         &num, int target) {        vector
        
         
          > res;        if(num.size()<4)return res;        sort(num.begin(),num.end());        int c,d;        for(int a=0;a
          
           0&&num[a]==num[a-1]) continue; for(int b=a+1;b
           
            a+1&&num[b]==num[b-1]) continue; c=b+1; d=num.size()-1; while(c
            
             b+1&&num[c]==num[c-1]) { c++; continue; } if(d
             
              target)d--; else { vector
              
                temp; temp.push_back(num[a]); temp.push_back(num[b]); temp.push_back(num[c]); temp.push_back(num[d]); res.push_back(temp); c++; d--; } } } } return res; }};