flatten.ex

defmodule Flattening do
  def flatten([]), do: []
  def flatten([head | tail]) when is_list(head), do: flatten(head) ++ flatten(tail)
  def flatten([head | tail]), do: [head | flatten(tail)]
end

IO.inspect Flattening.flatten([[1,2,[3]],[4]])
IO.inspect Flattening.flatten([[1,2,[3]],[4], [5]])
IO.inspect Flattening.flatten([[1,2,[3]],[4], [5, [[6]]]])
IO.inspect Flattening.flatten([[1,2,[3]],[4], [5, [[6]]], [7]])

# We take advatange of Elixir's linked lists and pattern matching.
# Recursion will ensure any lists in the head get flattened.
# It will recurse over the root and any nested lists until
# the head is not a list and the tail is an empty list because we reached the last element.
# The recursion creates dynamic branching which makes it very hard to visualize what's happening.