1
       |
   +---+---+---+
   |   |   |   |
   2   4   3   |
   |   |   |   |
   +---+---+   |
       |       |
       5       |
       |       |
       6       |
       |       |
       +---+---+
           |
           7
           |
           8
           |
       +---+---+
       |       |
       9       10
       +---+---+
           |
           11