Peter Winkler
Abstract
Last month (May 2014) we posted three puzzles in which you were asked to sort several cards using three stacks on a table; you were allowed to move the top card of one stack to the top of another (possibly empty) stack, with the object being to get all the cards in their natural order stacked in the leftmost place. The catch was you could see only the top cards of the stacks and had no memory. Not included were proofs that the algorithms described in the following solutions actually work; indeed, the best way to see how they work is to take three cards (or a whole suit) from a deck of playing cards and try.
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