i ^ (i >> 1), that's the general format

 

1 class Solution { 2 public: 3     vector
     
       grayCode(int n) { 4         vector
      
        result; 5         for (int i = 0; i < (1 << n); i++) { 6             result.push_back(i ^ (i >> 1)); 7         } 8         return result; 9     }10 };