[LeetCode] 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:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• 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> &num) {
        vector<vector<int> > res;
        int size = num.size();
        if(size < 3) return res;
        
        sort(num.begin(), num.end());
        
        for(int i=0;i<size-2;i++)
        {
            //thats key to avoid duplication!
            if(i && num[i] == num[i-1])continue;
            int start = i + 1;
            int end = size - 1;
            
            while(start < end)
            {
                int v = num[i]+num[start]+num[end];
                if(v < 0)
                {
                    start ++;
                }
                else if(v > 0)
                {
                    end --;
                }
                else
                {
                    vector<int> rec({num[i],num[start],num[end]});
                    res.push_back(rec);
                    start ++; end--;
      while (start<end&&num[start]==num[start - 1])
            start++;
                }
            }
        }
        return res;
    }
};