linked_list.rb

class LinkedList
  def initialize
    @head = @tail = nil
  end

  def add(value)
    node = Node.new(value)

    @head = node if @head.nil?
    @tail.next = node unless @tail.nil?

    @tail = node
  end
end

class Node
  def initialize(value)
    @value = value
    @next = nil
  end
  
  def next
    @next
  end

  def next=(value)
    @next = value
  end
end

list = LinkedList.new()
list.add(1)
# => current: 1, head: true, tail: true, next: nil
list.add(2)
# => current: 2, head: false, tail: true, next: nil
list.add(3)
# => current: 3, head: false, tail: true, next: nil

# Final Result
# Current: 1, head: true, tail: false, next: 2
# Current: 2, head: false, tail: false, next: 3
# Current: 3, head: false, tail: true, next: nil