How can I use sub-controller WITHOUT providing the directory name?

With routes.

PHP Code:

$route['someMethodA'] = 'subdirectory/ControllerA/someMethod';
$route['someMethodB'] = 'subdirectory/ControllerB/someMethod'; 


http://yoursite.com/someMethodA would then actually trigger /application/controllers/subdirectory/ControllerA/someMethod.