class Stack

  class EmptyError < StandardError
  end

  class Node
    attr_accessor :prev, :data
    def initialize(prev, data)
      @prev = prev
      @data = data
    end
  end

  def initialize
    @cur = nil
  end

  def empty?
    @cur.nil?
  end

  def push(val)
    if @cur
      new = Node.new(@cur, val)
      @cur = new
    else
      @cur = Node.new(nil, val)
    end
    val
  end

  def pop
    raise EmptyError if empty?
    popped = @cur
    @cur = @cur.prev
    popped.data
  end

  def size
    count(@cur, 0)
  end

  private

  def count(node, memo)
    return memo if node.nil?
    if node.prev
      count(node.prev, memo+1)
    else
      memo + 1
    end
  end

end