Personally, data structures. If you're using the correct data structure it doesn't mean you can't use a bad algo, but if you're using the wrong data structure i

Personally, data structures. If you're using the correct data structure it doesn't mean you can't use a bad algo, but if you're using the wrong data structure it guarantees you're not solving the problem efficiently. It's much easier to google "how to do X with graphs" to find some research on a great (if needed) runtime solution to your problem. It's really hard to google "how to sort these things so I can quickly do X with them" of course ... On top of that, if you have even a fairly-good data structure for your problem it makes coding and maintaining your solution that much more fun!