How can you detect a Cycle in a Linked List?

Constraint 1 : List is read-only, and you cannot mark elements.
Constraint 2 : There is limited amount of memory
Constraint 3 : The list is arbitarily long

Best Solution: (http://ostermiller.org/find_loop_singly_linked_list.html)

Catch Loops in Two Passes
O(n) time complexity

Simultaneously go through the list by ones (slow iterator) and by twos (fast iterator). If there is a loop the fast iterator will go around that loop twice as fast as the slow iterator. The fast iterator will lap the slow iterator within a single pass through the cycle. Detecting a loop is then just detecting that the slow iterator has been lapped by the fast iterator.

// Best solution
function boolean hasLoop(Node startNode){
Node slowNode = Node fastNode1 = Node fastNode2 = startNode;
while (slowNode && fastNode1 = fastNode2.next() && fastNode2 = fastNode1.next()){
if (slowNode == fastNode1 slowNode == fastNode2) return true;
slowNode = slowNode.next();
}
return false;
}