Reading the article, I too felt that another check was needed, to prevent the hare from sometimes leapfrogging eternally.<p>While drafting a snappy comeback to your comment that there was no such loop, I tried to find the smallest loop that demonstrated the bug, and came across a very nice pattern:<p>1: (1,1),(1,1)<p>2: (1,1),(1,2),(1,1)<p>3: (1,1),(3,2),(2,3),(1,1)<p>4: (1,1),(3,2),(1,3),(3,4),(1,1)<p>5: (1,1),(3,2),(5,3),(2,4),(4,5),(1,1)<p>6: (1,1),(3,2),(5,3),(1,4),(3,5),(5,6),(1,1)<p>...<p>Put another way, once the tortoise enters the loop, it will take &#60;=n-1 steps (where n is the size of the loop) for them to come together (without teleportation).<p>This problem seems like way too much fun to use in an interview!