Another interview question I just remembered:

1. If you had a website that you wanted to track how many people returned to the site to get some statistics on that, everyone on the site must login and there are log files that track every request and keep the user id and the session id. What type of data structures would you use and how would you insert, search, and select from it? What is the complexity of the chosen solution (Big O notation)? How would you keep a running percentage of the number of return visitors? A running percantage of return visitors in the past N days? etc.