In the above example, once we get to the second 3 node, how do we know we've got a cycle? We need some reference that we've previously encountered this vertex before.

What if we used a nested loop for tracking purposes? Let's take a look at what would happen in pseudocode:

xxxxxxxxxx

iterate through 1 -> 3 -> 5 -> 3 -> 7

  iterate through what we've seen so far