Communications of the ACM
Puzzled: Solutions and Sources
Credit: Merchant of Magic Ltd.
1. Three cards
Even though it would seem not too many things can be done with just three cards, there are actually 1.64 × 1018 memoryless algorithms to try, with most not working. You need brainpower. With some reasoning and experimentation, you may find that moving the cards mostly in one direction (such as to the right, including "around the corner" from the right stack to the left stack) helps avoid looping. Moreover, you generally want to expose more cards, except perhaps in some positions from which you might win. Of the several algorithms that do work, I like the following one, as suggested by my mathematics Ph.D. student Ewa Infeld, given by these four rules in priority order:
- Seeing 2,1,–, place 1 on 2;
- Otherwise, seeing two cards, move one card right (around the corner if necessary) to the open place;
- Seeing just one card, move that card to the left; and
- Seeing three cards, move the middle-rank card to the right.
2. Any number of cards.
This same algorithm works for any number of cards, numbered 1 through n. It takes quadratic time in the worst or random case, or approximately a constant times n2 steps.
Comments
Marc Auslander
I believe the following is quadratic and has not more than one card in the middle -
Call the stacks (and their top cards) A, B, and C
If B is empty:
if C is empty move A to B
elIf A is empty or A > C move C to A
else move C to B
If B is not empty:
If A is empty move B to A
If C is not empty and A > B > C move B to A
else move C to A
Intuitively, search left to right for an out of order card, then search right to left for a place to insert it.
Once all are sorted, the left to right search forces everything into the leftmost stack.
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