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