________                         ________
            |        |____                   |        |____    
      ______|             |            ______|             |   
     |                    |           |                    |   
 ____|                    |  =>   ____|    subproblem      | 
|                         |      |_________________________|   
|                         |      |     a single strip      |   
|_________________________|      |_________________________|