Write a method name greet that accepts a nullable Person as an argument and returns a String or null.

Depending on what kind of person it is, you should greet them differently:

• If the person is a Professor with name "Geoff" you should greet them "Hi Professor Geoff"
• If the person is a Student with name "Friendly" you should greet them "Hey Friendly, you are not alone!"
• If the person is a Staff, then they will have a String role property. If their role is "advising" and name is "Chuchu" you should greet them "Thanks Chuchu for all your help with advising".

All Persons have a name property. If the person is null or not one of the kinds of people described above, return null. Do not solve this problem using method overloading.