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>> res;
        if(nums.size() < 3)
            return res;

        sort(nums.begin(),nums.end());

        for(size_t i=0;i<nums.size();i++){
            int target = -nums[i];
            int front = i + 1;
            int back = nums.size()-1;
            while(front < back){
                if(nums[front] + nums[back] < target)
                    front++;
                else if(nums[front] + nums[back] > target)
                    back--;
                else{
                    res.push_back({nums[i],nums[front],nums[back]});
                    while(front < back && res.back()[1]==nums[front])
                        front++;
                    while(front < back && res.back()[2]==nums[back])
                        back--;
                }
            }
            while(i+1 < nums.size() && nums[i+1] == nums[i])
                i++;
        }
        return res;
    }
};