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There is an interesting interview question, which has been asked at many US companies.
There are 500 doors, initially closed. There are 500 people, one by one goes through all the doors.
The 1st person change the status of all the doors (i.e, open all of them).
The 2nd person change the status of all the doors with number can divide to 2 (i.e, he will change the status of doors number 2, 4, 6, 8, 10, ... 500)
The 3rd person change the status of doors 3, 6, 9, ...
...
The 500th person change the status of the door 500
Change the status means: close the opening door, and open the closing door.
The question is: in the end, how many doors are close?
You can solve this problem by using mathematics, but it may be faster to simulate it.
You can check my simulation at: http://www.codeskulptor.org/#user38_rrZv4nVRno_1.py and easy to see, the opening doors are the doors with the number are square number.
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