Gray Code

The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer A representing the total number of bits in the code, print the sequence of gray code.

A gray code sequence must begin with 0.

1 <= A <= 16

The first argument is an integer A.

Return an array of integers representing the gray code sequence.

Input 1:

A = 2

Input 1:

A = 1

output 1:

[0, 1, 3, 2]

output 2:

[0, 1]

Explanation 1:

for A = 2 the gray code sequence is:
    00 - 0
    01 - 1
    11 - 3
    10 - 2
So, return [0,1,3,2].

Explanation 1:

for A = 1 the gray code sequence is:
    00 - 0
    01 - 1
So, return [0, 1].