leetcode: 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.
序列排序方式是本题重点
二进制到格雷码转换规则，百度一下，你就知道
http://baike.baidu.com/view/358724.htm?from_id=11296193&type=syn&fromtitle=Gray+code&fr=aladdin

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