[LeetCode] 15. 3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> ret;
        
        sort(nums.begin(), nums.end());
        
        for (int i = 0; i < nums.size(); i++){
            // if i is  scanced already, skip it
            if (i > 0 && nums[i] == nums[i-1]) continue;
            int left = i + 1;
            int right = nums.size() - 1;
            while (left < right){
                if ((nums[left] + nums[right]) < -nums[i]){
                    left++;
                }else if ((nums[left] + nums[right]) > -nums[i]){
                    right--;
                }else{
                    vector<int> tmp;
                    tmp.push_back(nums[i]);
                    tmp.push_back(nums[left]);
                    tmp.push_back(nums[right]);
                    ret.push_back(tmp);
                    left++;
                    right--;
                    // remove duplicates!
                    while(nums[left] == nums[left - 1]) left++;
                    while(nums[right] == nums[right+1]) right--;        
                }
            }
        }
        
        return ret;
    }
};