LeetCode 18: 4Sum

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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:

  • Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie,abcd)
  • 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)


代码如下:

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


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