That's a really cool solution. One potential problem is that it doesn't find the loop as soon as possible (e.g. if it's going around the perimeter of a square,

That's a really cool solution. One potential problem is that it doesn't find the loop as soon as possible (e.g. if it's going around the perimeter of a square, the cycle is detectable after 4N steps, but the algorithm will take N^2 steps to realize that)