【优选算法】三数之和（双指针算法）

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums)
    {
        vector<vector<int>> ret;

        //1.排序
        sort(nums.begin(), nums.end());

        //2.利用双指针解决问题
        int n = nums.size();
        for (int i = 0; i < n; )
        {
            if (nums[i] > 0) break;
            int left = i + 1, right = n - 1, target = -nums[i];
            while (left < right)
            {
                int sum = nums[left] + nums[right];
                if (sum < target) left++;
                else if (sum > target) right--;
                else
                {
                    ret.push_back({ nums[i], nums[left], nums[right] });
                    left++, right--;
                    //去重 left 和 right，用left<right防止越界
                    while (left < right && nums[left] == nums[left - 1])
                        left++;
                    while (left < right && nums[right] == nums[right + 1])
                        right--;
                }
            }
            i++;
            //去重i,用i<n 来防止越界
            while (i < n && nums[i] == nums[i - 1]) i++;
        }
        return ret;
    }
};