算法优选：滑动窗口 1（单调性）

本期知识点导图

1. 上期参考代码

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        int n = nums.size();
        vector<vector<int>> ret{};
        for (int i = 0; i < n;) {
            for (int j = i + 1; j < n;) {
                int left = j + 1;
                int right = n - 1;
                long long aim = (long long)target - nums[i] - nums[j]; // 防止溢出
                while (left < right) {
                    int sum = nums[left] + nums[right];
                    if (sum < aim) left++;
                    else if (sum > aim) right--;
                    else {
                        ret.({nums[i], nums[j], nums[left++], nums[right--]});
                         (left < right && nums[left] == nums[left - ]) left++; 
                         (left < right && nums[right] == nums[right + ]) right--;
                    }
                }
                j++;
                 (j < n && nums[j - ] == nums[j]) j++; 
            }
            i++;
             (i < n && nums[i - ] == nums[i]) i++; 
        }
         ret;
    }
};