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
For a given n, a gray code sequence is not uniquely defined.
[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.