SiroDiaz / DataStructures

     * @param Callable|null $callback the callback function to apply to each

     *  node.

     */

    private function _preorder($node, Callable $callback = null) {

        if($node === null) {

            return;

        }

        if($callback !== null) {

            call_user_func($callback, $node);

        }

        $this->_preorder($node->left, $callback);

        $this->_preorder($node->right, $callback);

    }

    /**

     * Traverse in inorder. This is: first visit the left subtree,

     * @param Callable|null $callback the callback function to apply to each

     *  node.

     */

    private function _inorder($node, Callable $callback = null) {

        if($node === null) {

            return;

        }

        $this->_inorder($node->left, $callback);

        if($callback !== null) {

            call_user_func($callback, $node);

        }

        $this->_inorder($node->right, $callback);

    }

    /**

     * Traverse in postorder. This is: first visit the left subtree,

     * @param Callable|null $callback the callback function to apply to each

     *  node.

     */

    private function _postorder($node, Callable $callback = null) {

        if($node === null) {

            return;

        }

        $this->_postorder($node->left, $callback);

        $this->_postorder($node->right, $callback);

        if($callback !== null) {

            call_user_func($callback, $node);

        }

    }

}