Gray code (c++ code)

LeetCode Gray Code, May 20 '12
 难度4，出现频率2
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.

     vector<int> grayCode(int n) {
        vector<int> res;
        if(n == 0){ res.push_back(0); return res;}
        vector<int> last = grayCode(n-1);
        res.insert(res.end(), last.begin(), last.end());
        vector<int> :: iterator it = last.end() - 1;
        while( it != last.begin() - 1 ){
            res.push_back((1<<(n-1)) + *it);
            it--;
        }       
     
      return res;             

    }

注：如果不用recursive function call,直接写是这个样子：
(copy from UniEagle)

  vector<int> grayCode(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> ret;
        ret.push_back(0);
        if (n <= 0) return ret;
        int mostbit = 1;
        for(int i = 1; i <= n; ++i) {
            for(int k = ret.size() - 1; k >= 0; --k) {
                int v = ret[k];
                ret.push_back(v | mostbit);
            }
            mostbit = mostbit << 1;
        }
        return ret;
    }