传信问题
Message Delivery
题目详情
你需要通过一家快递服务与格林尼治的同事通信。文件会装在一个带挂锁的箱子里寄送。不幸的是，这家快递并不安全，因此任何装在未上锁箱子里的东西都会在运输途中丢失，包括你放在箱子里的锁。你和同事使用的高安全级别挂锁各自只有一把钥匙，且只有上锁的人持有这把钥匙。你该如何安全地把文件寄给同事？
You need to communicate with your colleague in Greenwich via a messenger service. Your documents are sent in a padlock box. Unfortunately the messenger service is not secure, so anything inside an unlocked box will be lost (including any locks you place inside the box) during the delivery. The high-security padlocks you and your colleague each use have only one key which the person placing the lock owns. How can you securely send a document to your colleague?
解析
先把文件放进箱子里，并用你的锁锁上，然后寄给同事。由于只有你有这把锁的钥匙，同事暂时打不开箱子。
同事收到后，不打开箱子，而是在箱子外再加上他自己的锁，然后把箱子寄回给你。此时箱子上有两把锁。
你收到后，用自己的钥匙把自己的锁去掉，再把箱子寄回给同事。现在箱子上只剩同事自己的锁。
最后，同事用自己的钥匙打开箱子，取出文件。
整个过程中，箱子始终至少被一把锁锁住，因此文件可以被安全传递。
Original Explanation
If you have a document to deliver, clearly you cannot deliver it in an unlocked box. So the first step is to deliver it to Greenwich in a locked box. Since you are the person who has the key to that lock, your colleague cannot open the box to get the document. Somehow you need to remove the lock before he can get the document, which means the box should be sent back to you before your colleague can get the document.
So what can he do before he sends back the box? He can place a second lock on the box, which he has the key to! Once the box is back to you, you remove your own lock and send the box back to your colleague. He opens his own lock and gets the document.