class Solution {
public:
    /**
     * @param n: a number
     * @return: Gray code
     */
    vector<int> grayCode(int n) {
        // write your code here
        vector<int> res(1<<n);
        for(int i =0 ; i < 1<<n; i++){
            res[i] = i ^(i>>1);
        }
        return res;
    }
};