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`