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From: anderson@uwvax.ARPA (David P. Anderson)
Newsgroups: net.puzzle
Subject: Solution to liar/truth-teller/diplomat problem
Message-ID: <406@uwvax.ARPA>
Date: Wed, 5-Sep-84 15:27:05 EDT
Article-I.D.: uwvax.406
Posted: Wed Sep  5 15:27:05 1984
Date-Received: Fri, 14-Sep-84 05:07:24 EDT
Organization: U of Wisconsin CS Dept
Lines: 35

<>
I have decided to *disallow* the solution posted by Eileen Rieback
because it involves questions which are yes/no/maybe rather than yes/no.
Eileen forfeits the $1000.00 first prize as a result.

Recall:
	truth-teller correctly answers all yes/no questions
	liar incorrectly answers all yes/no questions
	diplomat answers yes/no randomly
Your task: given one each of the above, ask 3 yes/no questions to
	figure out which is which.

SOLUTION: (call them X, Y and Z)

Ask X "Is your being a truth-teller equivalent to Y being a diplomat?"
If the answer is "yes"
    either X or Y is the diplomat
    Ask Z "Is your being a truth-teller equivalent to Y being a diplomat?"
    If the answer is "yes"
        Y is the diplomat
    else
        X is the diplomat
else
    either X or Z is the diplomat
    Ask Y "Is your being a truth-teller equivalent to X being a diplomat?"
    If the answer is "yes"
        X is the diplomat
    else
        Z is the diplomat

Now that you know who the diplomat is, you can distinguish the other two
by asking either of them a trivially true question such as "do you know
who the diplomat is?".

David Anderson (uwvax!anderson)