Three Switches

Question: You are standing outside a room next to three switches, all of which are off. Each switch operates a different light bulb in the room. The room door is closed, so you cannot see which switch operates which bulb. You are only allowed to go into the room once. Determine which switch operates which bulb.

Answer: Stumped? The issue lies in the fact that there are only two possible positions for each switch (on or off), but three bulbs to identify. Identifying one bulb is super easy, just set one switch on and then go into the room to check which one is on. But then, how do we detect the other two?

So let’s think deeper, what are the other properties of a light bulb operated by a switch? Light bulbs definitely produce light, but they also produce heat (a not so very important characteristic!). Any chance we can use this idea to help solve our problem? Since a light bulb would take sometime to lose its heat after being switched off, you could still determine if it had ever been on by touching it.

So now, we can easily determine which bulb is operated by which switch. First, we turn the first switch on and leave the other two off. After 5 minutes, we turn the first switch off, turn the second switch on and leave the third switch off. Then we go into the room and touch one of the bulbs which is turned off. After that, it would be easy to tell that the first switch matches the warm bulb, the second switch matches the bulb thats turned on and the third switch matches the colder bulb.

Many might think that this question would be a cheap shot at a trick question at an interview. Trust me, I thought the same. But when I was asked this question on an interview, my thoughts changed. So when prepping for a software interview, be weary of these kinds of questions!

If you're looking for some serious preparation for your interviews, I'd recommend this book written by a lead Google interviewer. It has 189 programming questions and solutions:

Book Cover

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