"""
Given a matrix of m * n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example:
Given the following matrix:
[
    [ 1, 2, 3 ],
    [ 4, 5, 6 ],
    [ 7, 8, 9 ]
]
You should return [1, 2, 3, 6, 9, 8, 7, 4, 5]
"""
