18

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