n数之和题目要类比——LeetCode题目18：四数之和

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

        if (nums.size() < 4) return res;
        
        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 l = j + 1, r = nums.size() - 1;
                while (l < r) {
                    int curr_sum = nums[i] + nums[j] + nums[l] + nums[r];
                    if (curr_sum == target) {
                        res.push_back({nums[i], nums[j], nums[l], nums[r]});
                        while ((l < r) && (nums[l] == nums[l+1])) ++l;
                        while ((l < r) && (nums[r] == nums[r-1])) --r;
                        ++l; --r;
                    }
                    else if (curr_sum > target) --r;
                    else ++l;
                }
            }
        }

        return res;
    }
};