Spiral traversal of a multidimensional list of characters

gistfile1.txt

# Given a multidimensional list of characters, print out a spiral traversal of the lists.
#
# iex(1)> matrix = [~w(A B C D), ~w(E F G H), ~w(I J K L), ~w(M N O P)]
# [
#   ["A", "B", "C", "D"],
#   ["E", "F", "G", "H"],
#   ["I", "J", "K", "L"],
#   ["M", "N", "O", "P"]
# ]
# iex(2)> Spiral.traverse(matrix, [])
# "A B C D H L P O N M I E F G K J"

defmodule Spiral do
  def traverse([], list), do: list |> Enum.join(" ")

  def traverse([row|[]], list) do
    list = list ++ row
    traverse([], list)
  end

  def traverse(matrix, list) do
    [row | matrix] = matrix
    list = list ++ row
    matrix
    |> transpose()
    |> traverse(list)
  end
  
  defp transpose(matrix) do
    matrix
    |> List.zip()
    |> Enum.map(&Tuple.to_list/1)
    |> Enum.reverse()
  end
end