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：这道题关键使用数学方法，公式表示：

（G：格雷码，B：二进制码）

理解了这个公式，代码就好写了。

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