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This is the puzzle that I started to tell last night, but didn't have
time for (as we left the restaurant).
There are four coins on a sheet of paper on the table, arranged in a
diamond. Call them North, South, East, and West. You are
blindfolded. You don't know which coins are heads or tails. Your
objective is to get the coins to all be the same way (ie, all heads or
all tails). On each turn, you may call out one or more directions
("East, North" for example), and your friend will flip those coins
over. The game ends when all coins face the same way. (Your friend
will tell you when the game is over). Also, after any of your moves,
your so-called friend may rotate the paper 90 degrees either way, or
180 degrees, thus redefining what north, south, east, west are. You
don't get to know whether the friend rotated the playing field, and at
no point do you get any information about which coins are heads or
tails.
What's your strategy for flipping the coins?
Surprisingly, there is enough information to consistently win the
game, in a fairly small number of moves.
----------------------------------------------------------------------
Eric J. Roode eric@myxa.com
Senior Software Engineer, Myxa Corporation
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