():87
                               |
                +--------------+--------+
                |                       |
              ():44                   ():43
        +-------+-------+         +-----+----+
        |               |         |          |
      ():20           ():24      e:21       f:22
   +----+----+     +----+----+
   |         |     |         |
  a:10      b:10  c:12      d:12