在PHP中实现二叉树的倒置

class TreeNode {
    public $val;
    public $left;
    public $right;

    public function __construct($val = 0, $left = null, $right = null) {
        $this->val = $val;
        $this->left = $left;
        $this->right = $right;
    }
}

function invertTree($root) {
    if ($root == null) {
        return null;
    }

    $temp = $root->left;
    $root->left = $root->right;
    $root->right = $temp;

    invertTree($root->left);
    invertTree($root->right);

    return $root;
}

// 示例用法
$root = new TreeNode(4);
$root->left = new TreeNode(2);
$root->right = new TreeNode(7);
$root->left->left = new TreeNode(1);
$root->left->right = new TreeNode(3);
$root->right->left = new TreeNode(6);
$root->right->right = new TreeNode(9);

$invertedTree = invertTree($root);