PHP 二叉树非递归遍历

<?php
    #二叉树的非递归遍历
    class Node {
        public $data;
        public $left;
        public $right;
    }

    #前序遍历，和深度遍历一样
    function preorder($root) {
        $stack = array();
        array_push($stack, $root);
        while (!empty($stack)) {
            $cnode = array_pop($stack);
            echo $cnode->data . " ";
            if ($cnode->right != null) array_push($stack, $cnode->right);
            if ($cnode->left != null) array_push($stack, $cnode->left);
        }
    }

    #中序遍历
    function inorder($root) {
        $stack = array();
        $cnode = $root;
        while (!empty($stack) || $cnode != null) {
            while ($cnode != null) {
                array_push($stack, $cnode);
                $cnode = $cnode->left;
            }

            $cnode = array_pop($stack);
            echo $cnode->data . " ";

            $cnode = $cnode->right;
        }
    }

    #后序遍历
    #使用双栈实现
    function postorder($root) {
        $pushstack = array();
        $visitstack = array();
        array_push($pushstack, $root);

        while (!empty($pushstack)) {
            $cnode = array_pop($pushstack);
            array_push($visitstack, $cnode);
            if ($cnode->left != null) array_push($pushstack, $cnode->left);
            if ($cnode->right != null) array_push($pushstack, $cnode->right);
        }

        while (!empty($visitstack)) {
            $cnode = array_pop($visitstack);
            echo $cnode->data . " ";
        }
    }

    $root = new Node();
    $n1 = new Node();
    $n11 = new Node();
    $n12 = new Node();
    $n2 = new Node();
    $n21 = new Node();
    $n22 = new Node();
    $root->data = 0;
    $n1->data = 1;
    $n11->data = 11;
    $n12->data = 12;
    $n2->data = 2;
    $n21->data = 21;
    $n22->data = 22;
    $root->left = $n1;
    $root->right = $n2;
    $n1->left = $n11;
    $n1->right = $n12;
    $n2->left = $n21;
    $n2->right = $n22;

    preorder($root);
    echo "<br>";
    inorder($root);
    echo "<br>";
    postorder($root);
?>