Hierarchical data structures can be very tricky but are necessary for threaded message boards and category/subcategory relationships. I spent countless hours learning how to implement them and really struggled through them when trying to create a message board. I really wish this tutorial existed at the time:

http://www.codeassembly.com/How-to-display-inifinit-depth-expandable-cat...

I found this little animated gif on a site.

Thought it was kinda cool - and in the interest of beating a horse to death - I decided to write up my own version of it. Here's a video of what i came up with:

Download the source for this app here.

So there's a web site, www.combinationlock.com, which I'm quite addicted to. It's a little online game that presents the player with a multi-reel combination lock. Each reel contains a number from 0-9, and a single combination of all the reels will open the lock -- just like a standard combination lock.

A typical game will present the player with a three-reel lock and a set of clues:

• The largest and smallest digits have a difference of 6
• Reel B has the highest digit
• Reel B is triple the last reel
• The answer is divisible by 2

In every case, the clues provided are enough to come up with one - and only one - correct answer. In this case, the answer is 062.

Just because I know it can be done, and I know it would be fun, I've set out to create an app which can parse the rules and crack the code.