Gray Code

描述
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:
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.

解法涉及到gray code的数学知识了，要是知道这个数学知识，可以在几分钟之内就解出这道题。
格雷码可以由对应的十进制数求出：grayCode=i^i>>1
runtime:4ms

class Solution {public:     vector<int> grayCode(int n) {         vector<int> result;         for(int i=0;i<1<<n;i++)//1<<n表示2的n次方。         {             result.push_back(i^i>>1);         }         return result;     }};