How to find whether a linked list is a circular linked list

[nuzzle]

but would this also work?

That's what I suggested and it will work. Sooner or later when you traverse a list you either reach the end or if it's circular or has a loop you come back to where you were before and that can be detected by storing all visited node pointers.

I wrote hashmap but I meant hashset. You really don't need the counters. You only need to know whether a node pointer is already present or not and for that a hashset is fine.

That said the other approach with two racing pointers at different speeds probably is better, although a little harder to get your head around. Here's a summary. The hashset solution is categorized as "correct but space inefficient",

http://ostermiller.org/find_loop_sin...nked_list.html