诚实者与说谎者
Knight and Knave
题目详情
旅行者在分岔路口遇到一对双胞胎：一条路正确，一条路错误。
其中一人永远说真话（诚实者），另一人永远说假话（说谎者）。旅行者可以向其中任意一人只问一个问题。
问：该问什么，才能确定哪条路是正确的？
提示：问其中一人“另一个人会怎么说”。
A traveler meets a pair of twins at a fork in the road: one path is correct and one incorrect. One of the twins always tells the truth (knight), and the other one always lies (knave). Which single question can the traveler ask one of the twins to determine the correct path?
Hint
Ask one twin about the other twin's knowledge!
解析
可以问：
“如果我问另一个人哪条路是正确的，他会指哪一条？”
无论你问到的是诚实者还是说谎者，对方给出的都会是“错误道路”。因此你选择相反的那条路即可。
Original Explanation
Solution
This can be constructed in many ways.
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The traveler can ask, "Would the other twin agree that the road on the left is correct?" The answer is guaranteed to be 'Yes' if and only if the road on the right is correct.
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The traveler can also ask, "If I were to ask you whether the road on the right is correct, would you say 'Yes'?", both the twins tell yes only if the right path is indeed correct.