Given 2 integers `n` and `start`. Your task is return any permutation `p` of `(0,1,2.....,2^n -1)` such that :

- `p[0] = start`
- `p[i]` and `p[i+1]` differ by only one bit in their binary representation.
- `p[0]` and `p[2^n -1]` must also differ by only one bit in their binary representation.

Example 1:

    Input: n = 2, start = 3

    Output: [3,2,0,1]

    Explanation: The binary representation of the permutation is (11,10,00,01).

    All the adjacent element differ by one bit. Another valid permutation is [3,1,0,2]

Example 2:

    Input: n = 3, start = 2

    Output: [2,6,7,5,4,0,1,3]

    Explanation: The binary representation of the permutation is (010,110,111,101,100,000,001,011).

Constraints:

- `1 <= n <= 16`
- `0 <= start < 2 ^ n`