PHP 函数中递归如何用于二叉树的遍历或操作？

class Node {
    public $data;
    public $left;
    public $right;

    public function __construct($data) {
        $this->data = $data;
        $this->left = null;
        $this->right = null;
    }
}

function preorderTraversal($root) {
    if ($root != null) {
        echo $root->data . " ";
        preorderTraversal($root->left);
        preorderTraversal($root->right);
    }
}

function inorderTraversal($root) {
    if ($root != null) {
        inorderTraversal($root->left);
        echo $root->data . " ";
        inorderTraversal($root->right);
    }
}

function postorderTraversal($root) {
    if ($root != null) {
        postorderTraversal($root->left);
        postorderTraversal($root->right);
        echo $root->data . " ";
    }
}

function findElement($root, $data) {
    if ($root == null) {
        return false;
    }

    if ($root->data == $data) {
        return true;
    }

    return findElement($root->left, $data) || findElement($root->right, $data);
}

function insertElement($root, $data) {
    if ($root == null) {
        return new Node($data);
    }

    if ($data < $root->data) {
        $root->left = insertElement($root->left, $data);
    } else {
        $root->right = insertElement($root->right, $data);
    }

    return $root;
}

            5
           / \
          3   7
         / \   \
        2   4   8

            5
          /   \
         3     7
        / \   / \
       2   4 1   8