该题与三数之和相同。

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> res;
        int n = nums.size();
        if (n < 3) return res;

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

        for (int i = 0; i < n; ++ i)
        {
            if (nums[i] > 0) return res;        //若第一个数大于0，加后面俩数就不可能小于0了
            if (i > 0 && nums[i] == nums[i - 1]) continue;  //跳过相同的数

            int left = i + 1, right = n - 1;    //第2,3个数采用双指针，一前一后
            while (left < right)
            {
                int curSum = nums[i] + nums[left] + nums[right];
                if (curSum == 0)
                {
                    res.push_back({nums[i], nums[left], nums[right]});
                    while (left < right && nums[left] == nums[left + 1]) ++ left;
                    while (left < right && nums[right] == nums[right - 1]) -- right;
                    ++ left, -- right;
                }
                else if (curSum > 0) -- right;  //结果大于0，右指针往左移
                else ++ left;                   //结果小于0，左指针往右移
            }
        }

        return res;
    }
};