LockersContributed by Robert Orenstein
A hallway in a school has 100 lockers. A student runs down the hall
and opens each locker. A second student runs down the hall and closes
every other locker. A third student runs down the hall and "changes the
state" of every third locker (i.e., if the locker is open, he closes it;
if it's closed, he opens it). This continues for 100 students -- the
N-th student changes the state of each N-th locker.