Loop in single linked list

Wow, 几天不见，这里正热闹，看来还是Progromers比Mathematician多。怪不得康MM被气得离家出走了。:)
既然大家对Programing感兴趣，给大家做一个。不过，哪位想Progrom也可以。
Any language is ok.

(An interview question)
Given a single linked list with n items.
Find an O(n) algorithm to detect any loops.