A Programming puzzle for students

It’s been quite a while since I’ve posted a programming challenge, that’s mainly because I haven’t heard a good one in a long time. I need my friends to do more interviews! Also, I’ve received emails in the past along the lines of “I really like the puzzles, and learn a lot from them, but can you post one a first year could solve”. That’s understandable, most of the puzzles I’ve posted previously might be tricky with only a few months of programming experience. So this programming puzzle is my attempt at monolithic di-ornithicide (aka killing two birds with one stone)

The longest repeated word

Write a method that takes a simple string as input, and outputs its longest repeated substring. When there are two of equal length, output whichever is first lexicographically.

Examples

• abba should return ’a’
• abracadabra should return ’abra’
• Mississippi should return ’iss’

You can get your solution to me any of the following ways, email, blogpost, trackback, IM, phonecall, even twitter it if your solution is <180 characters

Pretty please, I implore you, don’t post it as a comment You'll end up wondering why my blog ate all the rogue characters(< > ; etc). I can’t be bothered digging through 5K lines of someone elses PHP to try and stop it.