題目如下：

The gray code is a binary numeral system where two successive values differ in only one bit.

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.

十進位制       二進位制        格雷碼

0                   000             000

1                   001             001

2                   010             011

3                   011             010

4                   100             110

5                   101             111

6                   110             101

7                   111             100

二進位制轉格雷碼的規律為：先在二進位制最左邊新增0，舉例 010 -> 0010, 再前一位與後一位異或，得到後一位的格雷碼值，即：0^0=0; 0^1=1; 1^0=1; 因此得到格雷碼011。

該過程其實就相當於將二進位制右移一位，再與自身異或，舉例: 010 右移一位得到 001； 001^010=011。

所以解題思路就是:

（1）根據n得到所有10進位制的個數  1<<n

（2）對每個數，(i >>1)^i 即為i對應的格雷碼

程式碼如下：

public class Solution {
    public List<Integer> grayCode(int n) {
        List<Integer> list = new ArrayList<Integer>();
        int size = 1<< n;
        for(int i = 0; i < size; i++){
        	int greycode = (i >> 1) ^ i;
        	list.add(greycode);
        }
        return list;
    }
}