Interview Question for Software Development Engineer at Amazon.com:

Originally just presented as "a log of user web page accesses" I pressed him for the "time-ordered" part and made sure each transaction was marked with a user-specific ID. My solution was to split of the list by user, then assign three iterators to the first three values. Then use a three-tiered hashmap indexed by the first, second, then third page to store a sequence count. Each iteration we'd increment the value stored at map[pg1][pg2][pg3].