Code review week 5

[tjouni]

The code is well structured and commented. Some javadocs need updating, e.g. the TreeNode constructor has parameters leftChild and rightChild in javadoc, but not in the actual code.

Code is easy to read and I understood easily what it was supposed to do. Test coverage and tests in general look ok.

Great work in general!

Edited the part about strings, realised I understood the code wrong. Nice implementation, although I think in this course you are supposed to implement the substring method by yourself.