> By now you may be wondering why we do not simply use a set, and return fast if we encounter a node already in the set. Or, indeed, to solve what we’re building towards, we could just return the first value which is already in the set. (...)

> However, as I mentioned, this solution is spatially inefficient,

Typically you'd need to store N pointers, and the list would be of size N(size_of(pointer) + size_of(node)) - which would normally be much larger than the pointers? (otherwise use an array, perhaps?)

What are some real world cases where long linked list of very light nodes are best checked for cycles with this algorithm?