Most puzzles do not lend themselves to such simple solutions. This one is a rare exception in that the man has just one meaningful choice on all but the third trip The puzzle can also be solved by using the state-space graph (see [Lev06, Section 6.6]), similar to the solution of the Two Jealous Husbands puzzle in the tutorial on general design strategies. The states of the puzzle can also be represented by vertices of a cube. These alternative representations make it obvious that the seven trips is the fewest possible here. This classic puzzle was included in Alcuin’s collection—the earliest known collection of mathematical problems in Latin—which we already had a chance to mention in the first tutorial. On the appearance of this puzzle in other parts of the world, see [Ash90]. In modern times, it has become a standard feature in puzzle collections. Surprisingly, the puzzle still attracts the attention of mathematicians and computer scientists (see [Cso08]).
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