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]
]

vector<vector<int>> threeSum(vector<int>& nums) {
    vector<vector<int>> res;
    if (nums.size() < 3)return res;
    sort(nums.begin(), nums.end());

    for (int i = 0; i < nums.size() - 2; i++){      

        if (i > 0 && nums[i] == nums[i - 1]){
            continue;
        }

        int left = i + 1, right = nums.size() - 1;
        int prev = nums[i];

        while (left < right){
            if (prev + nums[left] + nums[right] > 0){
                right--;
                while (nums[right] == nums[right + 1])right--;
            }
            else if (prev + nums[left] + nums[right] < 0){
                left++;
                while (nums[left] == nums[left - 1])left++;
            }
            else{
                res.push_back({ prev, nums[left], nums[right] });
                left++, right--;
                while (nums[left] == nums[left - 1] && nums[right] == nums[right + 1])left++, right--;
            }
        }
    }
    return res;
}