class TaskList
  def items
    @items ||= []
  end
 
  def append(tree)
    items << tree
    tree.children.sort.each do |subtree|
      append subtree
    end
  end
end
 
class Task
  attr_reader :name, :children
 
  def initialize(name, children = [])
    @name = name
    @children = children
  end
 
  def <=>(other)
    name <=> other.name
  end
end
 
task_list = TaskList.new
 
task_list.append Task.new('Foo', [
  Task.new('Car', [
    Task.new('Ddd'),
    Task.new('Aaa'),
    Task.new('Ccc'),
    Task.new('Bbb'),
  ]),
  Task.new('Bar'),
])
 
puts task_list.items.map(&:name)

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