LeetCode18. 4Sum

题目：

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: 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]]

题目分析：

这道题和3Sum有点类似，但是不一样的是，这个要固定两个数。

具体的就不多解释了，可以看3sum的那道题。

代码：

class Solution {public:vector<vector<int>> fourSum(vector<int>& nums, int target) {vector<vector<int>> res;vector<int> tmp;set<vector<int>> sset;int begin, end;sort(nums.begin(), nums.end());if (nums.size() < 4) return res;for (int i = 0; i < nums.size() - 3; i++) {for (int j = i + 1; j < nums.size() - 2; j++) {begin = j + 1, end = nums.size() - 1;while (begin < end) {if (nums[i] + nums[j] + nums[begin] + nums[end] == target) {tmp.push_back(nums[i]);tmp.push_back(nums[j]);tmp.push_back(nums[begin]);tmp.push_back(nums[end]);sset.insert(tmp);tmp.clear();begin++;}else if (nums[i] + nums[j] + nums[begin] + nums[end] < target) {begin++;}else {end--;}}}}for (auto i : sset) res.push_back(i);return res;}};