LeetCode刷题（C++）——4Sum（Medium）

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]]

class Solution {public:    vector<vector<int>> fourSum(vector<int>& nums, int target) {        vector<vector<int>> res;if (nums.empty())return res;sort(nums.begin(), nums.end());int len = nums.size();for (int i = 0; i < len;i++) {for (int j = i + 1;j < len;j++) {int m = j + 1, n = len- 1;while (m < n) {    vector<int> v;int sum = nums[i] + nums[j] + nums[m] + nums[n];if (sum == target) {v.push_back(nums[i]);v.push_back(nums[j]);v.push_back(nums[m]);v.push_back(nums[n]);res.push_back(v);while (m + 1 < len&&nums[m] == nums[m + 1])++m;while (n > 0 && nums[n] == nums[n - 1])--n;++m;--n;}else if (sum < target) {while (m + 1 < len&&nums[m] == nums[m + 1])++m;++m;}else {while (n > 0 && nums[n] == nums[n - 1])--n;--n;}}while (j + 1 < len&&nums[j] == nums[j + 1])++j;}while (i + 1 < len&&nums[i] == nums[i + 1])++i;}return res;    }};