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 - 001 - 111 - 310 - 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.

二进制码->格雷码（编码）：从最右边一位起，依次将每一位与左边一位异或（XOR），作为对应格雷码该位的值，最左边一位不变（相当于左边是0）；

格雷码->二进制码（解码）：从左边第二位起，将每位与左边一位解码后的值异或，作为该位解码后的值（最左边一位依然不变）。

Gray Code 0 = 0, 下一项是toggle最右边的bit(LSB), 再下一项是toggle最右边值为 “1” bit的左边一个bit，然后重复。直到最右边值为 “1” 的bit在最左边了，结束。

class Solution {
public:
    vector<int> grayCode(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> result;
        int nSize = 1 << n;
        for (int i = 0; i < nSize; ++i)
        {
            result.push_back((i>>1)^i);
        }
        return result;
    }
};