Competitive programming in Haskell: summer series

Now that this weird and crazy semester is finally over, I’d like to get back to doing some blogging (among other things). I’m going to return to my series on competitive programming in Haskell (previous posts here: basic setup; code style and moral absolutes; Scanner; ByteString; primes and factoring; modular arithmetic 1; modular arithmetic 2), but following a more hands-on approach: each Tuesday and Friday, I will post a link to a problem (or two), and invite you to try solving it. The following Tuesday or Friday, I will explain my solution (as well as link to another problem for the next post). Sometimes I may develop a topic over several posts; sometimes I may just post random problems that I like.

To kick things off, here’s an easy-ish problem with an elegant Haskell solution:

I invite you to try solving it, and to post your questions, comments, and solutions here! (Fair warning: don’t look at the comments before you’ve solved it if you don’t want spoilers!) On Tuesday I will post my solution with commentary, and another problem for you to try.