题目描述：

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.

考察位操作：

从题目中例子可以发现这么一个规律：

假设结果数组为B，原始数组为A，则有：

B[i]  = A[i] ^ (A[i]>>1).  

例如 ： A[3] =3     
       B[3] = 3^(3>>1) 
            = 011^001 
            = 010 = 2

解法如下：

 1 class Solution {
 2 public:
 3     vector<int> grayCode(int n) {
 4         int len = pow(2.0,n),i,a;
 5         vector<int> vi;
 6         for(i=0;i<len;i++)
 7         {
 8             a= i^(i>>1);
 9             vi.push_back(a);
10         }
11         return vi;
12     }
13 };