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:
给定一个数组，找出所有4个数的组合使得它们的和为target
思路：和3Sum，Two Sum都是类似的。主要是两个点：
1. 对数组排序，然后双指针分别从头尾遍历
2. 注意去重的问题
1 class Solution { 2 public: 3 vector<vector<int>> fourSum(vector<int>& nums, int target) { 4 int nsize = nums.size(); 5 sort(nums.begin(), nums.end()); 6 vector<vector<int>> result; 7 for(int i = 0; i < nsize; i++){ 8 if(i != 0 && nums[i] == nums[i - 1]) 9 continue; 10 for(int j = i + 1; j < nsize; j++){ 11 if(j != i + 1 && nums[j] == nums[j - 1]) 12 continue; 13 int p = j + 1, q = nsize - 1; 14 while(p < q){ 15 int sum = nums[i] + nums[j] + nums[p] + nums[q]; 16 if(sum < target){ 17 ++p; 18 continue; 19 } 20 if(sum > target){ 21 --q; 22 continue; 23 } 24 25 vector<int> tmp; 26 tmp.push_back(nums[i]); 27 tmp.push_back(nums[j]); 28 tmp.push_back(nums[p]); 29 tmp.push_back(nums[q]); 30 result.push_back(tmp); 31 32 while(++p < q && nums[p] == nums[p - 1]); 33 while(p < --q && nums[q] == nums[q + 1]); 34 } 35 } 36 } 37 return result; 38 } 39 };
原文：http://www.cnblogs.com/coolqiyu/p/5330983.html