題目鏈接：https://leetcode.com/PRoblems/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 - 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.
For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.
 解題思路：
手動地列出n=3時候的情況，會發現。n=3的結果如下:
000001011010  ↑--------110  ↓111101100
可看成是上下對稱的2部分。上半部分是n=2時候的結果再在最高位加"0"的結果。下半部分是n=2時候的結果逆序之后再在最高位加"1"的結果。
class Solution{public:    vector<int> grayCode(int n)    {        vector<int> res;        res.push_back(0);        if(n==0)            return res;        for(int i=1;i<=n;i++)        {            for(int j=res.size()-1;j>=0;j--)            {                res.push_back(res[j]+pow(2,i-1));            }        }        return res;    }};