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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] ]
1 class Solution { 2 public: 3 vector<vector<int>> fourSum(vector<int>& nums, int target) { 4 vector<vector<int>> ret; 5 6 sort(nums.begin(), nums.end()); 7 8 // for i and j, need to avoid duplicates. 9 for (int i = 0; i < nums.size(); i++){ 10 if (i > 0 && nums[i] == nums[i-1]) continue; 11 for (int j = i + 1; j < nums.size(); j++){ 12 // here j > .... 13 if (j > i + 1 && nums[j] == nums[j-1]) continue; 14 int left = j + 1; 15 int right = nums.size() - 1; 16 while (left < right){ 17 int tmp = nums[i] + nums[j] + nums[left] + nums[right]; 18 if (tmp < target){ 19 left++; 20 }else if (tmp > target){ 21 right--; 22 }else{ 23 vector<int> sol; 24 sol.push_back(nums[i]); 25 sol.push_back(nums[j]); 26 sol.push_back(nums[left]); 27 sol.push_back(nums[right]); 28 ret.push_back(sol); 29 left++; 30 right--; 31 while(nums[left] == nums[left-1]) left++; 32 while(nums[right] == nums[right+1]) right--; 33 } 34 } 35 } 36 } 37 38 return ret; 39 } 40 };
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原文地址：http://www.cnblogs.com/amadis/p/5926507.html
