PHP 二叉樹的深度優先與廣度優先遍歷

<?php
     #二叉樹的廣度優先遍歷
     #使用一個隊列實現
 
     class Node {
         public $data = null;
         public $left = null;
         public $right = null;
     }
 
     #@param $btree 二叉樹根節點
     function breadth_first_traverse($btree) {
         $traverse_data = array();
         $queue = array();
         array_unshift($queue, $btree); #根節點入隊
         
         while (!empty($queue)) { #持續輸出節點，直到隊列為空
             $cnode = array_pop($queue); #隊尾元素出隊
             $traverse_data[] = $cnode->data;
 
             #左節點先入隊，然后右節點入隊
             if ($cnode->left != null) array_unshift($queue, $cnode->left);
             if ($cnode->right != null) array_unshift($queue, $cnode->right);
         }
 
         return $traverse_data;
     }

     #深度優先遍歷,使用一個棧實現
     function depth_first_traverse($btree) {
        $traverse_data = array();
        $stack = array();
        array_push($stack, $btree);

        while (!empty($stack)) {
            $cnode = array_pop($stack);
            $traverse_data[] = $cnode->data;

            if ($cnode->right != null) array_push($stack, $cnode->right);
            if ($cnode->left != null) array_push($stack, $cnode->left);
        }

        return $traverse_data;
     }
 
     $root = new Node();
     $node1 = new Node();
     $node2 = new Node();
     $node3 = new Node();
     $node4 = new Node();
     $node5 = new Node();
     $node6 = new Node();
 
     $root->data = 1;
     $node1->data = 2;
     $node2->data = 3;
     $node3->data = 4;
     $node4->data = 5;
     $node5->data = 6;
     $node6->data = 7;
 
     $root->left = $node1;
     $root->right = $node2;
     $node1->left = $node3;
     $node1->right = $node4;
     $node2->left = $node5;
     $node2->right = $node6;
 
     $traverse = breadth_first_traverse($root);
     print_r($traverse);
     echo "<br>";
     $traverse = depth_first_traverse($root);
     print_r($traverse);
 ?>

Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 [5] => 6 [6] => 7 ) 
Array ( [0] => 1 [1] => 2 [2] => 4 [3] => 5 [4] => 3 [5] => 6 [6] => 7 )