PHP 创建二叉树镜像(交换左右子树)

<?php
    #生成二叉树镜像(交换树的所有左右节点)

    class Node {
        public $data = null;
        public $parent = null;
        public $left = null;
        public $right = null;
    }

    #递归方法
    function mirror_recursive($root) {
        if ($root === null) {
            return;
        }

        mirror_recursive($root->left);
        mirror_recursive($root->right);
        $temp = $root->left;
        $root->left = $root->right;
        $root->right = $temp;
    }

    #非递归方法,用一个栈或队列都可以,类似于深度优先和广度优先遍历
    function mirror_norec($root) {
        $queue = array();
        array_unshift($queue, $root);

        while (!empty($queue)) {
            $cnode = array_pop($queue);
            if ($cnode->left !== null) array_unshift($queue, $cnode->left);
            if ($cnode->right !== null) array_unshift($queue, $cnode->right);
            $temp = $cnode->left;
            $cnode->left = $cnode->right;
            $cnode->right = $temp;
        }
    }

    #中序遍历
    function inorder_traverse($root) {
        if ($root->left !== null) inorder_traverse($root->left);
        echo $root->data . " ";
        if ($root->right !== null) inorder_traverse($root->right);
    }

    $root = new Node();
    $n1 = new Node();
    $n2 = new Node();
    $n11 = new Node();
    $n12 = new Node();
    $n13 = new Node();
    $n14 = new Node();
    $n15 = new Node();
    $n21 = new Node();

    $root->data = 0;
    $n1->data = 1;
    $n2->data = 2;
    $n11->data = 11;
    $n12->data = 12;
    $n13->data = 13;
    $n14->data = 14;
    $n15->data = 15;
    $n21->data = 21;

    $root->left = $n1;
    $root->right = $n2;
    $n1->left = $n11;
    $n1->right = $n12;
    $n11->left = $n13;
    $n12->right = $n14;
    $n13->left = $n15;
    $n2->right = $n21;

    inorder_traverse($root);
    echo "<br>";
    mirror_recursive($root);
    inorder_traverse($root);
    echo "<br>";
    mirror_norec($root);
    inorder_traverse($root);
?>

15 13 11 1 12 14 0 2 21 
21 2 0 14 12 1 11 13 15 
15 13 11 1 12 14 0 2 21