Reddit has 3 or so weekly challenges (rated easy, intermediate, and hard) located at: http://www.reddit.com/r/dailyprogrammer
I've decided to start doing some of these and will be placing my solutions here.
I generally will solve these using Groovy. My goal is not to pre- or over-optimize or go for the fewest lines of code, but instead I try to make readable code with comments so others can follow the code. If a 3rd party library makes sense, I don't have any issues with using that as part of the solution, assuming that the library doesn't wholesale provide the solution (if the goal is to write a HashMap, I won't just import a HashMap and call it done).