Sudoku

http://norvig.com/sudoku.html An essay by Peter Norvig showing how to program sudoku solver. Note: Read this if you are a software developer . Even better is to print it out and read it with a pencil.

http://www.infoq.com/news/2007/05/tdd-sudoku an article (with a lot of useful links, make sure you follow them) about how Ron Jeffries (author of XP programming) solves sudoku algo in a series of 5 blog articles to show his TDD approach. This is was contrasted by Peter Norvig solving the same problem ( http://norvig.com/sudoku.html ) , but in a more clear and concise way.