This isn't thinking outside the box, it's an incredibly obvious way to avoid using your brain. There's nothing wrong with creating constraints on a problem, as

This isn't thinking outside the box, it's an incredibly obvious way to avoid using your brain. There's nothing wrong with creating constraints on a problem, as long as those are clearly defined. Presumably the OP didn't neglect to mention that strings were disallowed, and then just fail anyone who happened to use them.