Two Pointers or Iterators

threeSum.cpp

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 l = i+1;
        int r = nums.size()-1;
        int val = -nums[i];
        while(l < r){
            if(nums[l] + nums[r] > val) r--;
            else if(nums[l] + nums[r] < val) l++;
            else{
                res.push_back({nums[i], nums[l++], nums[r--]});
                while(l < r && nums[l] == nums[l-1]) l++;
                while(l < r && nums[r] == nums[r+1]) r--;
            }
        }
    }
    return res;
}