After being roasted by so many interviewers, I have found one common point in all these interviews and this point is the more interesting part that no one would believe it at the first sight. If I have to reveal the secret, I have no one to reveal because by the time I have observed this, almost all my friends were placed and I feel M4apt is a better place for me to reveal this observation as it helps lot of people like me.
In the beginning, I thought that when an interviewer asks me a puzzle, he expects a correct answer for the same and that too within less time and he asks it because he want to test my solving ability and how fast I'm able to do it. But that's not the case, In my 1st interview, the interviewer asked me a puzzle for which I have no clue. I tried all the possible ways and was able to make halfway through. Now the interviewer motivated me to do it for 10 more minutes but I wasn't able to do and I gave up. The interviewer then gave me a small hint "what you did in your previous step? think again" and there was the solution, I have to apply the same logic for this step too. I have said the answer and I still had a question mark of getting selected or not as I already took soo many hints.
After a few weeks, results are out and I got selected. [Didn't think about the interview, forgot it]
A few months later, the same thing happened with another company, I took soo many hints, gave up and they helped me with few more hints. finally, I got placed in that company too.
Why does an interviewer ask a puzzle?
They want to test your ability of thinking and they don't expect 100% correct answer in less time rather they want to observe how many ways you are trying to approach the solution and how creatively you think and how much time you are trying without any hesitation.
with my personal experience in many interviews, I can tell you one thing, be quiet cool and try to approach the solution in multiple ways. In case if you are not able to proceed further or if you are stuck, they will be happy to give you some or many hints. ultimately they expect you to come to a conclusion or solution with their help.
what if you know the answer?
If you already know the answer, don't tell it immediately rather take some time and try to think in many other ways and finally come to the solution after a few moments.
should you really search for the puzzles that are more probable to be asked?
In my words, a big "NO", go with a mood to solve but not with the answers list. you need not search for puzzles before attempting an interview, just be natural.