flatten.rb

class SmallTool
  def initialize
    @result_array = []
  end

  def array_flatten(target_array)
    target_array.each do |item|
      if item.is_a?(Array)
        array_flatten(item)
      else
        @result_array << item
      end
    end
    @result_array
  end
end

# puts "#{SmallTool.new.array_flatten([[1, 2, [3]], 4])}"

test_flatten.rb

require 'test/unit'
require_relative 'flatten'

class TestFlatten < Test::Unit::TestCase
  def setup
    @small_tool = SmallTool.new
  end

  def test_flatten
    assert_equal([1, 2, 3, 4], @small_tool.array_flatten([[1, 2, [3]], 4]))
  end
end