gistfile1.rb

class ListNode
  attr_accessor :prev, :next, :value

  def initialize(prev, next, value)
    @prev = prev
    @next = next
    @value = value
  end
end

class List
  attr_accessor :tail, :head

  def initialize(value)
    node = ListNode.new nil, nil, value
    @head = node
    @tail = node
  end
end