Programming Homework Help

I posted whole package here for my assignment. Because I got poor grade from first two assignment, I really need to get better grade on this project.

Assignment Instruction is also uploaded in document file.
The thing that I need to working on is Tree.java file. You will see the empty part, which says /** complete this method **/. These are the only parts that needed to be completed. You must not change any other codes except for /** complete this method **/.

I after finishing it I just need completed Tree.java file. I am also going to study with completed version, so I want the codes as easy as possible.
Thanks for your help,