15. 3Sum

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> res;
        if (nums.size() == 0)return res;
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size() - 1; i++){
            int j = i + 1, k = nums.size() - 1;
            while (j < k) {
                if (nums[i] + nums[j] + nums[k] == 0) {
                    res.push_back({nums[i], nums[j], nums[k]});
                    while (j < k && nums[j] == nums[j + 1]) j++;
                    while (j < k && nums[k - 1] == nums[k]) k--;
                    j++;
                    k--;      ///please don't forget this!!!!!!!!!
                }
                else {
                    if (nums[i] + nums[j] + nums[k] > 0) k--;
                        else j++;
                }
            }
            while (i < nums.size() - 1 && nums[i] == nums[i + 1]) i++;
        }
        return res;
    }
};