class CategoryTree
  attr_reader :id
  attr_reader :name
  attr_reader :left
  attr_reader :right

  def initialize id, name, left=nil, right=nil
    @id, @name, @left, @right = id, name, left, right
  end
end

def traverse tree, level = 0
  puts "-- "*level + tree.id.to_s + " " + tree.name
  traverse tree.left, level+1 if tree.left
  traverse tree.right, level+1 if tree.right
end

tree = CategoryTree.new 1, "a", CategoryTree.new(2, "b"), CategoryTree.new(22, "c", CategoryTree.new(3, "d"))

traverse tree