Detect a loop in cyclic/circular linked list

Logic :

If a linked list has a cycle then no node has a null as next pointer.
A simple loop exists when p1.next.next !== null as shown below.

Simplest possible loop

While iterating over the linked list we need two pointers, p1 increments one node at a time and the p2 increments two points at a time. If both the pointers meet then the loop exists.

If there is no loop then p1 and p2 can not meet as p2 is in the future, we have not invented time travel yet :)

Circular Linked List iteration for the given solution

Watch the following video to understand the Floyd's cycle-finding algorithm!

This post is a follow-up of JavaScript Linked List Example. I recommend reading those posts first, as the following code uses the method from it.

Solution :

References :

Stackoverflow
Floyd's algorithm for cycle detection

linked list 6 singly-linked-list 9 linked 2 circular 3 cyclic 3 loop 1