15. 3Sum

https://leetcode.com/problems/3sum/description/

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> res;
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size(); i++) {
            if (i > 0 && nums[i] == nums[i-1])  continue;
            for (int j = i + 1, k = nums.size() - 1; j < k; ) {
                int t = nums[j] + nums[k];
                if (t == -nums[i]) {    // found
                    auto cur = vector<int>({nums[i], nums[j], nums[k]});
                    res.push_back(cur);
                    
                    j++; k--;
                    while (j < k && nums[j] == nums[j-1]) j++;
                    while (j < k && nums[k] == nums[k+1]) k--;
                }
                else if (t > -nums[i])
                    k--;
                else
                    j++;
            }
        }
        return res;
    }
};