It's nowhere near as fascinating as some other stories here. But earlier this year I and a friend created an Android app for our school schedules. Initially we thought it would be easy; "let's ask the schools devs for an API or something".

Turns out it's a basically a black box they've bought from an obscure company.

We then needed to parse the HTML tables by hand, which should be relatively straight-forward for simple tables. But then it turns out the tables are filled with rowspans, colspans and nested tables. It took me quite a while to create a decent algorithm, as I had never done anything similar before. But now we can parse all the HTML tables on that website and cache them in the app.

As I said, nothing impressive or fascinating, but I was still quite proud of my algorithm.