算法描述：
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.
Example 1:
Input: 2
Output: [0,1,3,2]
Explanation:
00 - 0
01 - 1
11 - 3
10 - 2
For a given n, a gray code sequence may not be uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence.
00 - 0
10 - 2
11 - 3
01 - 1
Example 2:
Input: 0
Output: [0]
Explanation: We define the gray code sequence to begin with 0.
A gray code sequence of n has size = 2n, which for n = 0 the size is 20 = 1.
Therefore, for n = 0 the gray code sequence is [0].
解题思路：最高位或1.
0
1 // 第一轮0 | 1 << 0
00
01
11 //第二轮 保留第一轮原有数字，并添加新数字--即在现有数字最高位或1
10
000
010
011
010
100 //第三轮 保留第二轮所有数字，并添加新数字--数字最高位或1
110
111
110
从后往前添加是为了不让后面添加的数对原有元素进行干扰。
vector<int> grayCode(int n) { vector<int> results; results.push_back(0); for(int i=0; i < n; i++){ for(int j=results.size()-1; j >= 0; j--){ results.push_back(results[j] | 1 << i); } } return results; }