leetcode_Gray Code

描述：

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.
思路：

1.n位格雷码对应有1<<n个数

2.每个数字对应的格雷码的对应的计算公式为：i^(i/2)

代码：

public List<Integer> grayCode(int n) {
List<Integer>list=new ArrayList<Integer>();
if(n<0)
return list;
n=1<<n;
for(int i=0;i<n;i++)
list.add((i^(i/2)));
return list;
}