list.rb

class Node
  attr_accessor :next
  attr_accessor :value

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

class List
  def initialize
    @head = nil
    @last = nil
  end

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

    if @head.nil?
      @head = node
      @last = node
    else
      @last.next = node
      @last = node
    end
  end

  def enum
    enumerator = Enumerator.new do |e|
      node = @head
      until node.nil?
        e << node.value
        node = node.next
      end
    end

    enumerator
  end

  def inspect
    enum.to_a.inspect
  end

  def reverse!
    return if @head.nil?

    head = @last

    while @head != head
      element = @head
      @head = @head.next

      second = head.next
      head.next = element

      element.next = second
    end
  end
end

# a,b,c,d,e ; head = e ; head.next = nil, @head = a
# b,c,d,e,a ; head = e ; head.next = a, a.next = nil, @head = b
# c,d,e,b,a ; head = e ; head.next = b, b.next = a, @head = c
# d,e,c,b,a ; head = e ; head.next = c, c.next = b, @head = d
# e,d,c,b,a ; head = e ; head.next = d, d.next = c ; @head = e, @head == head