An intelligence test question for Microsoft interviews -- Wang Zhifeng

Microsoft is well known in the industry for its use of quiz questions during interviews. In addition to many interesting questions, difficulties, and strange questions used by Microsoft, there are also many interesting stories related to problem solving and answers. I personally experienced one.

The story happened when I joined Microsoft and worked at the headquarters of the Dell Computer Company in Austin, Texas before I joined Microsoft. One day, a female colleague in my group did not know where to get an intelligence test question. She told me mysteriously that this question was just appeared in Microsoft's interview recently. It is said that the average time for Microsoft internal staff to solve the problem is less than five minutes, and the average level of external experts is 15 minutes. However, many external staff cannot solve the problem, no matter how long it takes. The female colleague said she had thought for a long time and had no results, so let me try. I am usually not interested in the intelligence test questions, but I heard her say it was so mysterious, and she was excited by her, so I took the question seriously and finally solved the answer within five minutes, she was admired. The story is half done here. Before the second half of the story, I have to explain the questions and answers to you:

The question is: There are four people (A, B, C, and D) who want to spend a long night in the dark of a month. A bridge can only carry two people at a time, that is, a maximum of two people can cross the bridge at a time. The bridge uses a flashlight, and the four have only one flashlight. That is to say, after the two share the flashlight, one of them must bring the flashlight back (no other way ), otherwise, no one else can proceed. Because of the differences in age and physical condition, each person needs to spend different time crossing the bridge: A needs 1 minute, B needs 2 minutes, C needs 5 minutes, and D needs 10 minutes. Because a flashlight is shared, when two people bridge the bridge together, the bridge time is based on one of the slow ones. For example, it takes 5 minutes for A and C to bridge the bridge together.

Now, how many minutes does it take for everyone to bridge the bridge?

This problem does not seem complex, it is easy to start with, and the problem-solving strategy is not difficult to determine. Since the flashlight needs to be passed back and forth, according to the principle of "more effort", we certainly try to use the fastest person to carry on this task. Because a bridge is the fastest, the following answer will be obtained soon:

Step 1: A and B bridge the bridge together. The time is 2 minutes.

Step 2: A returns with a flashlight for 1 minute

Step 3: A and C bridge together, time is 5 minutes

Step 4: A returns with a flashlight for 1 minute

Step 5: A and D bridge over the bridge. The time is 10 minutes.

19 minutes in total

This seems reasonable, but the answer is wrong. The correct answer is 17 minutes. The readers may wish to think about how to get this answer.

In fact, the key to solving the problem is whether you can think of another more important strategy: to let the slowest person (C and D) bridge the bridge together. Please try again based on this principle.

The following are the correct answers:

Step 1: A and B bridge the bridge together. The time is 2 minutes.

Step 2: A returns with a flashlight for 1 minute

Step 3: C and D bridge over the bridge. The time is 10 minutes.

Step 4: B returns with a flashlight for 2 minutes

Step 5: A and B bridge the bridge in 2 minutes

17 minutes in total

It can be seen that the key to this question is whether you can break through the obvious mindset, find more effective methods that are not easy to find, and sometimes dare to violate that obvious rule.

Well, let's go back to the second half of the story. While the female colleague was amazed, she suddenly thought

I want to try another expert. So she thought of her boyfriend. Her boyfriend graduated from the Department of Physics from Texas University, where she worked on chip design at the Motorola division in Austin. The female immediately picked up the phone and told her boyfriend the question. In less than two minutes, her boyfriend called back and said that the answer had been removed. The answer took 19 minutes. We all laughed at the scene and told him that the answer was wrong. It should take a shorter time for him to think about it again. After hanging up the phone for more than 10 minutes, her boyfriend's phone rang again. This time we thought that he had made the correct answer. Instead, we did not expect that he did not make the correct answer. Instead, he said, 19 minutes is the absolute shortest time, he used mathematical methods to prove this conclusion theoretically! After listening, we laughed.

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