Solution:
+     +     +     +     +     +     +
                                     
               1           1     0   
                                     
+     +-----+-----+-----+-----+     +
      |                       |      
      |  3           2        |      
      |                       |      
+     +-----+-----+-----+     +     +
                        |     |      
   0     1           2  |  3  |  1   
                        |     |      
+     +     +     +     +-----+     +
                                     
               0           1         
                                     
+     +     +     +     +     +     +