Sample solutions.

* The solutions for p1-1, p1-2, and p2 are okay.

* The solution for p1-3 needs some work.  It doesn't pass all of the
  tests.

* The solution for p3 is terrible.  For example, there's no locking at
  all.  I just wrote it to make sure that the project was possible
  given the tools that we gave the students.