[apendleton]

Project Euler is great for practicing this sort of thing. With the early questions, you can solve them however and it's fine, but many of the later problems have an obvious quadratic (or worse) solution that ends up being too slow in practice to finish in a sane amount of time, and it forces you to come up with a better algorithm to actually solve the puzzle. As a bonus, once you solve it you get to see other people's solutions, which are often super clever.