data structure exercise 01

This is a simple library circulation management system. A library has a number of patrons, each of which has a five digit ID number. The library's holdings are books, identified by their titles. Assuming that titles are unique, and that the library holds only one copy of each title.

Patrons can borrow and return titles, and can request a list of what they've borrowed. The library can also find out which patron has borrowed a given title. Waiting lists can exist for popular books. The library can get a list of patrons who have ever borrowed a book, even if they have nothing checked out at the moment.