3_sum.cpp

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(begin(nums), end(nums));
        vector<vector<int>> res;
        for (int i = 0; i < nums.size() && nums[i] <= 0; ++i)
            if (i == 0 || nums[i - 1] != nums[i]) {
                twoSumII(nums, i, res);
            }
        return res;
    }
    void twoSumII(vector<int>& nums, int i, vector<vector<int>> &res) {
        int lo = i + 1, hi = nums.size() - 1;
        while (lo < hi) {
            int sum = nums[i] + nums[lo] + nums[hi];
            if (sum < 0) {
                ++lo;
            } else if (sum > 0) {
                --hi;
            } else {
                res.push_back({ nums[i], nums[lo++], nums[hi--] });
                while (lo < hi && nums[lo] == nums[lo - 1])
                    ++lo;
                while (lo < hi && nums[hi+1] == nums[hi])
                    --hi;

            }
        }
    }
};