Loop in single linked list

Wow, 幾天不見，這裏正熱鬧，看來還是Progromers比Mathematician多。怪不得康MM被氣得離家出走了。:)
既然大家對Programing感興趣，給大家做一個。不過，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.