Permutations

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

class Solution {
public:
    vector<vector< int> > permute(vector< int> &num) 
    {
         bool used[num.size()];
         for( int i= 0;i<num.size();i++)
            used[i]= false;
        vector<vector< int> > result;
        vector< int> possible;
         for( int i= 0;i<num.size();i++)
            possible.push_back( 0);
        gen(result,num,used, 0,possible);
         return result;
    }
     void gen(vector<vector< int> >& result,vector< int>& num, bool* used, int step,vector< int>& possible)
    {
         if(step==num.size())
        {
            result.push_back(possible);
             return;
        }
        
         for( int i= 0;i<num.size();i++)
         if(!used[i])
        {
            used[i]= true;
            possible[step]=num[i];
            gen(result,num,used,step+ 1,possible);
            used[i]= false;
        }
    }
};