18. 4Sum
class Solution { public: vector<vector<int>> fourSum(vector<int>& nums, int target) { vector<vector<int>> res; if(nums.size()<4){ return res; } vector<int> vec; sort(nums.begin(),nums.end()); for(int i=0;i<nums.size()-3;i++){ if(i>0 && nums[i] == nums[i-1]){ continue; } for(int j=i+1;j<nums.size()-2;j++){ if(j>i+1 && nums[j] == nums[j-1]){ continue; } int m=j+1,n=nums.size()-1; int sum = target-nums[i]-nums[j]; while(m<n){ if(nums[m]+nums[n]<sum){ m++; } else if(nums[m]+nums[n]>sum){ n--; } else{ vec.clear(); vec.push_back(nums[i]); vec.push_back(nums[j]); vec.push_back(nums[m]); vec.push_back(nums[n]); res.push_back(vec); m++; n--; while(m<n && nums[m] == nums[m-1]) m++; while(m<n && nums[n] == nums[n+1]) n--; } } } } return res; } };