find all unique triplets in array with sum equal to zero in c++

Hello I have refered this program and there mention in comment // DO NOT write int main() function, and when i run run this code it will give me error:- /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../lib64/crt1.o: In function _start': (.text+0x20): undefined reference tomain' collect2: ld returned 1 exit status

I wrote main for it bu still it wont compile please suggest any suggestion for this issue. Thanx in advanced. I wrote my code as below:-

#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > vv;
        if(num.size()<3){
            return vv;
        }

        sort(num.begin(), num.end());
        // In general, we can make the triples unique by the set trick
        // For 3Sum, it is guaranteed by 1, 2, 3
        for(int i = 0; i<num.size()-2; i++){
            if(i>0 && num[i]==num[i-1]){                // 1
                continue;
            }
            // convert 3Sum to 2Sum
            int target = -num[i];
            if(target<0){
                break;
            }
            int j = i+1;
            int k = num.size()-1;
            while(j<k){
                if(num[j]+num[k]==target){
                    vector<int> v;
                    v.push_back(num[i]);
                    v.push_back(num[j]);
                    v.push_back(num[k]);
                    vv.push_back(v);
                }
                if(num[j]+num[k]<=target){


                    // Don't forget ++  


                    while(num[j++]==num[j] && j<k){};   // 2
                }
                else{
                    while(num[k--]==num[k] && k>j){};   // 3
                }
            }
        }
        return vv;
    }
};