Spiral Matrix

Problem

Given a matrix of m x 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].