问题描述：

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：

vector<vector<int> > fourSum(vector<int> &num, int target) {  //C++
        vector<vector<int> > res; 
        if (num.size() <= 3) return res;
        sort(num.begin(), num.end());
        int twoSum;
        for (int i = 0; i < num.size() - 3;)
        {
            for(int j = i+1; j < num.size()-2;){
                int l = j+1, r = num.size() - 1;
                twoSum = target -num[i] - num[j];
                while (l < r)
                {
                    if (num[l] + num[r] < twoSum) l++;
                    else if (num[l] + num[r] == twoSum)
                    {
                        vector<int> three(4);
                        three[0] = num[i];
                        three[1] = num[j];
                        three[2] = num[l];
                        three[3] = num[r];
                        res.push_back(three);
                        do { l++; }while (l < r && num[l - 1] == num[l]);
                        do { r--; }while (l < r && num[r + 1] == num[r]);
                    }
                    else r--;
                }
                do{ j++; }while (j < num.size() - 2 && num[j - 1] == num[j]);
            }
            do{ i++; }while (i < num.size() - 3 && num[i - 1] == num[i]);
        }
        sort(res.begin(),res.end());
        return res;
    }

[leetcode] 18 4Sum