flatten.rb

class CB
  def self.flatten(arr)
    flat_array = []
    arr.each do |e|
      if e.is_a?(Array)
        CB.flatten(e).each {|se| flat_array << se }
      else
        flat_array << e
      end
    end
    flat_array
  end
end

flatten_test.rb

require 'minitest/autorun'
require_relative 'flatten'

describe CB do
  describe "#flatten" do
    it "should flatten an array of arbitary nested arrays" do
      CB.flatten([]).must_equal []
      CB.flatten([1, 2, 3]).must_equal [1, 2, 3]
      CB.flatten([1, [1,2], 3]).must_equal [1,1, 2, 3]
      CB.flatten([1, [1, [1, 2]], 3]).must_equal [1, 1, 1, 2, 3]
      CB.flatten([1, [[5, [6, [7, [8, 9]]]],2],3]).must_equal [1, 5, 6, 7, 8, 9, 2, 3]
    end
  end
end