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;
        }//如果大小小于4，就可以直接返回了
        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] > target - nums[i + 2] - nums[i + 3] ) {
                break;

//这里这么写是为了不溢出，四个数相加会可能会溢出

            }
            if( nums[i] + nums[n - 1] < target - nums[n - 2] - nums[n - 3] ) {
                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] > target - nums[j + 1] - nums[j + 2]) {
                    break;
                }
                if(nums[i] + nums[j] < target - nums[n - 1] - nums[n - 2]) {
                    continue;
                }               
                int left = j + 1, right = n - 1;
                while(left < right) {
                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
                    if(sum == target) {
                        ans.push_back({nums[i], nums[j], nums[left], nums[right]});
                        while( (left < right) && (nums[left] == nums[left + 1]) ){
                            left++;
                        }
                        left++;
                        while( (left < right) && (nums[right] == nums[right - 1]) ){ 
                            right--;
                        }
                        right--;
                    }
                    else if(sum < target) {
                        left++;
                    }
                    else {
                        right--;
                    }
                }               
            }            
        }
        return ans;
    }
};