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)

思路：还是在枚举了两个数后利用双指针发来求解

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        vector<vector<int>> res;
        res.clear();
        int len = num.size();
        if (len < 4) return res;

        sort(num.begin(), num.end());
        set<vector<int>> ans;

        for (int i = 0; i < len - 3; i++) {
            for (int j = i+1; j < len; j++) {
                int s = j + 1;
                int e = len - 1;
                while (s < e) {
                    int sum = num[i] + num[j] + num[s] + num[e];
                    if (sum == target) {
                        vector<int> tmp;
                        tmp.push_back(num[i]);
                        tmp.push_back(num[j]);
                        tmp.push_back(num[s]);
                        tmp.push_back(num[e]);
                        ans.insert(tmp);
                        s++;
                        e--;
                    } else if (sum > target) {
                        e--;
                    } else {
                        s++;
                    }
                }
            }
        }

        set<vector<int>>::iterator it = ans.begin();
        for (; it != ans.end(); it++)
            res.push_back(*it);
        return res;
    }
};

LeetCode 4Sum