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

 

We add 2^i to the corresponding number in the reversed copy of the previous array, then append it to the array every time.

class Solution:    # @return a list of integers    def grayCode(self, n):        a = [0]        for i in range(n):            a += [x + 2**i for x in a[::-1]]        return a