flatten.rb

require 'test/unit'

def flatten(array)
  flattened_array = []

  array.each { |item|
    if item.is_a?(Array)
      flattened_array += flatten(item)
    else
      flattened_array << item
    end
  }

  flattened_array
end

class FlattenTest < Test::Unit::TestCase
  def test_flatten_basic_array
    assert_equal([1, 2, 3], flatten([1, 2, 3]))
  end

  def test_flatten_one_inner_array
    assert_equal([4, 5, 6, 7, 8], flatten([4, 5, 6, [7, 8]]))
  end

  def test_flatten_two_inner_arrays
    assert_equal([1, 2, 3, 4, 5], flatten([1, 2, [3, [4, 5]]]))
  end

  def test_flatten_example_given
    assert_equal([1, 2, 3, 4], flatten([[1,2,[3]],4]))
  end
end