[Illustration]
A man has twenty-five dog kennels all communicating with each other by
doorways, as shown in the illustration. He wishes to arrange his twenty
dogs so that they shall form a knight's string from dog No. 1 to dog No.
20, the bottom row of five kennels to be left empty, as at present. This
is to be done by moving one dog at a time into a vacant kennel. The dogs
are well trained to obedience, and may be trusted to remain in the
kennels in which they are placed, except that if two are placed in the
same kennel together they will fight it out to the death. How is the
puzzle to be solved in the fewest possible moves without two dogs ever
being together?