Here's one possible solution for the riddle I posted a few days ago:

1. Replace i < N with -i < N
2. Replace i-- with N--
3. Replace i < N with i + N

This isn't a particularly hard riddle - it took me about 10 minutes to come up with the solution, although I imagine a developer with more recent C experience will be much quicker. Regardless it's a pretty good way to tone those C muscles.