output

Node(1)
  Node(2)
    Node(3)
    Node(4)
      Node(5)
  Node(6)
  Node(7)
    Node(8)

tree.rb

class Node

  @@counter = 0

  attr_reader :parent, :children

  def initialize(parent=nil, children=nil)
    @parent = parent
    @children = children || []
    @@counter += 1
    @num = @@counter
  end

  def root?
    parent.nil?
  end

  def parent=(node)
    raise "Not a node!" unless node.is_a? Node
    @parent = node
    node.children << self
  end

  def <<(node)
    raise "Not a node!" unless node.is_a? Node
    node.parent = self
  end

  def inspect
    "#<#{name} parent=#{parent.inspect}, children=#{children.inspect}>"
  end

  def name
    "Node(#{@num})"
  end

  def walk(level=0, &block)
    yield [self, level]

    children.each do |child|
      child.walk(level+1, &block)
    end
  end

  def draw
    walk do |node, level|
      puts "  "*level + node.name
    end
  end

end


root =  Node.new\
        << (
          Node.new\
            << Node.new\
                << (
                    Node.new\
                      << Node.new
                   )
           )\
        << Node.new\
          << (
            Node.new\
              << Node.new
             )

root.draw