add questions Q9.7, Q9.9
Q18.13 working..

Change-Id: Icc2eeb9a23c809822ff7fb49d43fb2a1f7a8ab94
Added question to Q4.1
Incomplete answer to Q4.2 find route between two nodes in directed graph
organize imports

Change-Id: I8e390fe9d2b5133ae8464d146030e1fb9160d5f3
complete solution to Q4.2
answer to Q4.4

Change-Id: Iec88ed8feded2e072e5a6eee49d51ab8bd4ac353
Check BST

Change-Id: I8e9392f2739b12e399d081e96d540c9627418f64
Answer to Q4.5
Updated answers to Q4.4, Q4.5
added overloaded factory method BSTNode.node()

Change-Id: Ie4aaac69e275b4d531396cb0872c58881e0cfaa7