和之前的3Sum和3SumCloset是一个思路，随便写写就行。

class Solution {
public:
	vector<vector<int>> fourSum(vector<int>& nums, int target) {
		int len = size(nums);
		sort(nums.begin(),nums.end());

		vector<vector<int>> re;

		if (len < 4 || nums[0]>0 || nums[len - 1] < 0) return re;
		if (len == 4) {
			if (nums[0] + nums[1] + nums[2] + nums[3] == target) re.push_back(nums);
			return re;
		}

		int mar_l = 0;
		int mar_r = len - 1;
		for (; mar_l < len-1; mar_l++) {
			if (mar_l>0&&nums[mar_l-1] == nums[mar_l])continue;
			mar_r = len - 1;
			for (; mar_l < mar_r; mar_r--) {
				if (mar_r<len-1&&nums[mar_r+1] == nums[mar_r])continue;
				int in_l = mar_l + 1;
				int in_r = mar_r - 1;
				int temp_mar = -(nums[mar_r] + nums[mar_l]) + target;
				while (in_l != in_r&& in_l < in_r) {
					int temp_in = nums[in_l] + nums[in_r];
					if (temp_in == temp_mar) {
						re.push_back({ nums[mar_l],nums[in_l],nums[in_r],nums[mar_r] });
                        in_l++;
                        in_r--;
						while (in_l<in_r && nums[in_l - 1] == nums[in_l]){
							in_l++;
						}
						while (in_l<in_r && nums[in_r + 1] == nums[in_r]){
						    in_r--;
                        }
					}
					else if (temp_in > temp_mar) {
						in_r--;
					}
					else {
						in_l++;
					}
				}
			}
		}
		return re;
	}
};