89. Gray Code

##题目
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
这道题目，答案不唯一，本文给出其中一种解法，采用按位操作，即异或的使用，实现巧妙，理解简单
##代码

class Solution {
public:
    vector<int> grayCode(int n) {
    vector<int> result;
    for (int i = 0; i < 1<<n; i++) result.push_back(i ^ i>>1);//异或操作
    return result;
    }
};