public function __construct(AbstractBinaryTree $root = NULL)
 {
     $this->stack = new \SplStack();
     parent::__construct($root);
 }
 public function __construct(AbstractBinaryTree $root = NULL)
 {
     parent::__construct($root);
 }
 public function __construct(AbstractBinaryTree $root = NULL)
 {
     $this->queue = new \SplQueue();
     parent::__construct($root);
 }