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.
	 * 
	 * */