Gray Code

/*

Gray Code

The gray code is a binary numeral system where two successive values differ in only one bit.
Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.
For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 0
01 - 1
11 - 3
10 - 2

Note:
For a given n, a gray code sequence is not uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.
For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

*/
class Solution {
public:
    int pow(int base, int n){
       if(n==0) return 1;
       if(n==1) return base;
       return base*pow(base, n-1);
    }
   
    void getGrayCode(int n, vector<int> & result){
       if(n<0) return;
       if(n==0) {
          result.push_back(0);
          return;
       }
      
       getGrayCode(n-1, result);
       int size = result.size();
       int i;
       for(i=size-1; i>=0; i--){
         result.push_back(result[i]+pow(2,n-1));
       }
       return;
    }
    vector<int> grayCode(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> result;
        if(n<0) return result;
        getGrayCode(n, result);
        return result;
    }
};