Gray Code

An n-bit gray code sequence is a sequence of 2^n integers where:

• Every integer is in the inclusive range [0, 2^n - 1]
• The first integer is 0
• An integer appears no more than once in the sequence
• The binary representation of every pair of adjacent integers differs by exactly one bit
• The binary representation of the first and last integers also differ by exactly one bit

Given an integer n, return any valid n-bit gray code sequence.

Examples

Example 1:

Input: n = 2

Output: [0,1,3,2]

Explanation:

• 00 - 0
• 01 - 1
• 11 - 3
• 10 - 2

Example 2:

Input: n = 1

Output: [0,1]

Constraints

• 1 <= n <= 16