Gray Code

class Solution {public:    vector<int> grayCode(int n) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<int> res;        if (n==0) {            res.push_back(0);            return res;        }        //int k= log(n)/log(2);        res.push_back(0);        res.push_back(1);        int i=1;        while (i<=n-1) {             int n=res.size()-1;            for (int j=n; j>=0; j--) {                res.push_back(res[j]+exp2(i));            }             i++;        }        return res;    }};