Treeのオブジェクトに対してeachでごにょごにょしたいときのテンプレート的な何か

tree_iterator_in_ruby.rb

class Tree
  attr_accessor :val, :level, :l, :r
  def initialize(val, level, l, r)
    @val = val; @level = level; @l = l; @r = r
  end
  def each
    stack = []
    stack.push self
    explored = []
    while !stack.empty?
      u = stack.shift
      if !explored.include?(u)
        explored.push u
        yield u
        stack.unshift u.l unless u.l.nil?
        stack.unshift u.r unless u.r.nil?
      end
    end
  end
end

tree = Tree.new("World", 0,
                Tree.new("Asia", 1,
                         Tree.new("Japan", 2, nil, nil),
                         Tree.new("China", 2, nil, nil)),
                Tree.new("Europe", 1, 
                         Tree.new("Dutch", 2, nil, nil),
                         Tree.new("Portugal", 2, nil, nil)))

tree.each {|node|
  puts "\s" * node.level + node.val
}