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Tech-Interview Riddles
An anonymous submitter writes "A computer engineering student at UC Berkeley has made a comprehensive archive of riddles from technical interviews. Very challenging and loads of fun. Also useful for interview preparation."
for more tech interview questions and answers try http://www.techinterview.org/
The "official" answer (which Microsoft was still using as recently as a couple of years ago, according to some friends of mine who interviewed there) is so the covers won't fall through the hole.
This answer fails on at least two levels:
1) There are plenty of manhole covers (or manhole-cover-like-objects) that aren't round. If you've been observant enough to notice this, you fail.
2) There are plenty of other curves of constant width; an infinite number, in fact. The old Wankel rotary engine used such a shape. Though a circle is the only curve of constant *radius*, that's not the issue. If you know enough math to realize this, you fail.
Another possible answer is that it makes it easier to roll a heavy cover out of the way. Again, one of the other curves of constant width would do just about as well.
The REAL answer is that no one knows.
Personally, I think Microsoft would be better off asking people why using fixed-sized buffers for user input is a bad idea, but hey.....
Well, crikey. But ask yourself this - if not round, well, what?
Okay, so you're a guy, and you have to put a hole in the street, and put a cover on it. What shape do you make it? Triangular? Well, uh, why? It's pointy, and can fall through the hole. So you wouldn't do that. Square? Well, you really can't roll it, and TRUCKS have to drive over it, so it'll be heavy, so you'd *want* to roll it, rather than heft it.
Hey, circles can roll.
Oh, yeah. And that crap about a Wankel. Why would you want to fiddle around in traffic trying to get it to be oriented properly when a circle HAS a constant radius, as you point out? You can thump it down any old way. Fits!
So, answer: Because it's just better than any other shape.
Some answers from the hard section:
.2 with an average payoff of $5. That means that the probability of getting a face card is .8 with a payoff of 11.5. Using more precise figures, i.e. not .2 and .8, the average payoff is about 10.0857 (706/70)
Criminal cupbearers:
Let's assume we only have 10 prisoners and that they each drink from up to 512 bottles. Number the bottles from 0 to 999. Prisoner 9 samples 0 to 511. Prisoner 8 samples 0 to 255 and 512 to 999. Prisoner 7 samples 0 to 127, 256 to 383, 512 to 639, etc. (prisoners alternating between sampling and not sampling blocks of wine in decreasing powers of 2 -- prisoner 0 drinks from every other bottle) Now line up the prisoners after onen month and treat corpses as ones and living prisoners as zeros and you have your answer in binary.
Mysterious Triangle area
Well, to make a long story short, they're not triangles.
100 Prisoners and a Lightbulb
Well if we assume they can all see the bulb every day, they can just toggle the bulb iff this is the first time they've been selected. If the last prisoner has counted the number of times the bulb has been toggled, he can assert that he is the last one to be selected.
Square Formation
Move the "notched" piece to teh righth of the current larger square and put the small square piece in the notch. put the larger of the triangular pieces at the top, horizontal edge of the new formation.
Calendar Cubes
I like this one. You need all the numbers from 0 through 9 plus 0 through 3. That's 14 faces. You will never need 00 though, so you can remove one of the 0s. Also, you will only ever need the 3 with 0 or 1, so you can remove it from one of the blocks. The solution: the numbers 1-6 on one block, and 7-9 and 0-2 on the other. Yeah it works.
Mystery Matrix
4. Entry from row plus Entry from row 2 plus 1 mod 10.
Fork in the road I
"is that the city you come from?" If the response is yes, go there, otherwise turn away.
Fork in the road II
Assume each person is standing on his respective road. "Is one of you a liar?" Yes means he's a truth teller, no means he's a liar.
Egg Dropping
18. Drop from the 10th, 20th, 30th, etc. After it breaks, go back 9 floors and start dropping every floor. You use 18 drops if it can drop from the 98th or 99th floors.
Greedy Pirates
It's not apparent to me that this is the intended answer, but "Throw pirates 3 and 4 overboard and divide up the rest between 1,2, and 5. Pirates 1 and 2 will agree to the largest share, and pirate 5 always has a say after that, since 3 and 4 can't agree to anything, so he's needed for the majority.
Hmm, well it's getting late so I'll just do one more:
Card Game
Bob takes any card over 9. The probability that none will show up is roughly
I did a lot of interviews at MS when I was there, and I quickly learned not to ask riddles. First off, it makes people who don't get them uncomfortable and angry. Second, it doesn't actually show that the person can write software.
I used a much simpler approach, so simple most people think its silly. But thats the point; nobody leaves the interview thinking they were tricked or duped. I always started with implementation of strcpy(). Half of the candidates failed right there! They took most of the hour to get it right (or not), but were able to see point-blank that they were not ready for the job.
Next, I would ask about crashing cases, and if they figured out overlapping memory locations, have them write a 'fixed' version. This weeded out another big chunk. After that, I went into some color counting algorithms.
I stayed well withing the field of what the candidate would expect, and did not try to trick him or make him nervous with off the wall riddles.
This approach worked great, and didn't leave anyone feeling robbed and abused. The ability to solve riddles *is* an indicator of how smart the person is, but it is *not* an indicator of how good a programmer they will be.