LeetCode 18 四数之和

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;

        sort(nums.begin(), nums.end());

        for (int i = 0; i < nums.size(); i ++) {
            if (nums[i] >= 0 && nums[i] > target) break;
            if (i > 0 && nums[i] == nums[i - 1]) continue; //去重i

            for (int k = i + 1; k < nums.size(); k ++) {
                if (nums[k] + nums[i] >= 0 && nums[k] + nums[i] > target) break;
                if (k > i + 1 && nums[k] == nums[k - 1]) continue; //去重k

                int left = k + 1, right = nums.size() - 1;
                while (left < right) {
                    if ((long)nums[i] + nums[k] + nums[left] + nums[right] > target) right --;
                    else if  ((long)nums[i] + nums[k] + nums[left] + nums[right] < target) left ++;
                    else {
                        res.push_back({nums[i], nums[k], nums[left], nums[right]});

                        while (left < right && nums[left] == nums[left + 1]) left ++; //去重left
                        while (left < right && nums[right] == nums[right - 1]) right --; //去重right

                        left ++;
                        right --;
                    }
                }
            }
        }

        return res;
    }
};