A singly linked list with unknown length, how to quickly find the element in the middle?

A singly linked list with unknown length, how to quickly find the element in the middle?

// Only ideas
// http://justjavac.iteye.com/blog/1497457
// Set two pointers, p1, p2, and start p1 and p2 at the head of the link.
// p1 takes two steps each time,
// p2 takes one step each time
// When p1 reaches the end of the linked list, the position of p2 is the middle element of the linked list