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.

思路：

这题其实挺简单，思路可以联想到组合问题中的回溯，改变第n位，然后再利用其余n-1位的组合重新组成n位的组合。如果在改变第n位的时候，保持其余n-1位不动，再保持第n位将其余n-1位的序列反着推回去就OK了。

感觉程序没有写出来的必要，举个栗子反而更清楚 n = 4

0 0 0 0
0 0 0 1
0 0 1 |1
0 0 1 |0

0 1 |1 0
0 1 |1 1
0 1 |0 1
0 1 |0 0

1 |1 0 0
1 |1 0 1
1 |1 1 1
1 |1 1 0

1 |0 1 0
1 |0 1 1
1 |0 0 1
1 |0 0 0