Try reading the docs first its very simple :
'sample' => array(
'type' => 'Segment',
'options' => array(
'route' => '/:category[/:sub_category[/:id]].html',
'defaults' => array(
'controller' => 'Your Controller',
'action' => 'Your Action',
),
),
),
With this router config you can have :
http://www.example.com/category.html
http://www.example.com/category/sub-category.html
http://www.example.com/category/sub-category/id.html