Gray Code

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

• Every integer is in the inclusive range [0, 2n - 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, and
• The binary representation of the first and last integers differs by exactly one bit.

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

Example 1:

Input: n = 2
Output: [0,1,3,2]
Explanation:
The binary representation of [0,1,3,2] is [00,01,11,10].

• 00 and 01 differ by one bit
• 01 and 11 differ by one bit
• 11 and 10 differ by one bit
• 10 and 00 differ by one bit [0,2,3,1] is also a valid gray code sequence, whose binary representation is [00,10,11,01].
• 00 and 10 differ by one bit
• 10 and 11 differ by one bit
• 11 and 01 differ by one bit
• 01 and 00 differ by one bit

Example 2:

Input: n = 1
Output: [0,1]

Constraints:

• 1 <= n <= 16

SOLUTION:

class Solution:
    def grayCode(self, n: int) -> List[int]:
        if n == 1:
            return [0, 1]
        prev = self.grayCode(n - 1)
        k = 1 << (n - 1)
        return prev + [prev[i] + k for i in range(k - 1, -1, -1)]