Check If the Singly Linked List has a Loop

The list shown above contains a loop – from 5 its connected to 2. There are several incorrect and inefficient methods of finding a loop. The most efficient method is discussed here.

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.
This solution is "Floyd’s Cycle-Finding Algorithm" as published in "Non-deterministic Algorithms" by Robert W. Floyd in 1967. It is also called "The Tortoise and the Hare Algorithm". Further Reading on various approaches.
This algorithm has – O(n) time complexity

// 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;
}