89 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

Solution:

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int> res;
        if ( n == 0 ) {
            res.push_back(0);
            return res;
        }
        res = grayCode(n-1);
        int nres = res.size();
        for ( int i = nres-1; i >= 0; i-- ) res.push_back(res[i] + pow(2, n-1));
        return res;
    }
};

Question: iterative solution?