15. 3sum.cpp

class Solution {
public:
vector<vector<int>> threeSum(vector<int> &nums)
{
    sort(nums.begin(), nums.end());
    if (nums.size() < 3)
    { 
        return {};
    }
    if (nums[0] > 0)
    { 
        return {};
    }
    vector<vector<int>> answer;
    for (int i = 0; i < nums.size(); ++i)
    { 
        if (nums[i] > 0)
        { 
            break;
        }
        if (i > 0 && nums[i] == nums[i - 1])
        { 
            continue;
        }
        int low = i + 1, high = nums.size() - 1; 
        int sum = 0;
        while (low < high)
        {
            sum = nums[i] + nums[low] + nums[high];
            if (sum > 0)
            {
                high--;
            }
            else if (sum < 0)
            {
                low++;
            }
            else
            {
                answer.push_back({nums[i], nums[low], nums[high]});
                int last_low_occurence = nums[low], last_high_occurence = nums[high]; 
                while (low < high && nums[low] == last_low_occurence)
                { 
                    low++;
                }
                while (low < high && nums[high] == last_high_occurence)
                {
                    high--;
                }
            }
        }
    }
    return answer; 
}
};