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