Gray Code

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.

分析：返回格雷码。二进制数转为格雷码的过程为：从左至右，保留第一位，将二进制数第一位或上二进制第二位得到格雷码第二位，二进制第二位或上二进制第三位得到格雷码第三位....以此类推。比如，二进制数1010，首先保留1，然后将1或上0得到1, 0或上1得到1， 1或上0得到0，所以结果为1110。

其实说白了就是a ^ (a >> 1).

运行时间：5ms

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