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.

公式：

G(i) = i^ (i/2).

 class Solution {

     public List<Integer> grayCode(int n) {

         List<Integer> res = new ArrayList<Integer>();

         for(int i =0;i<1<<n;i++)

             res.add(i^i>>1);

         return res;

     }

 }