标签：
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) { int l = nums.size(), sum, left, right, i, j; vector<vector<int>> ans; if (l<4) return ans; sort(nums.begin(), nums.end()); for (i = 0; i<l - 3; i++) { if(i && nums[i] == nums[i-1]) continue; for (j = i + 1; j<l - 2; j++) { if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target) break; if(nums[i]+nums[j]+nums[l-2]+nums[l-1]<target) continue; if (j>i + 1 && nums[j] == nums[j - 1]) continue; sum = nums[i] + nums[j]; left = j + 1; right = l - 1; while (left < right) { if (sum + nums[left] + nums[right] == target) ans.push_back({ nums[i], nums[j], nums[left++], nums[right--] }); else if (sum + nums[left] + nums[right] < target) left++; else right--; while (left>j+1 && nums[left] == nums[left - 1]) left++; while(right<l-1 && nums[right] == nums[right + 1]) right--; } } } return ans; } };
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原文地址：http://www.cnblogs.com/argenbarbie/p/5804043.html
