List of Puzzles

It is very common these days to be asked a couple of puzzles during technical interviews. Following is a list of few puzzles that can be asked during your interview. These puzzles are not solved and the list is compiled just to provide you an overview of what might be asked. You can search on the web about different puzzles and there solution.

Question 1:

There is a disk on a turntable. Half of it is black, and the other half is white. It is rotating at constant speed, but it's rotating fast so that you can't see whether it's clockwise or counterclockwise. You also have a set of stationary sensors you can install anywhere which can tell whether the color of a spot has changed from black to white, or vice versa.
What is the minimum number of sensors needed to find out which way the disk is rotating?
What is the best, average, worst time to figure out the direction?
If you use more sensors, can you improve the best, average, worst time?
How can you accomplish that?
If you are given only one sensor, how can you find out the direction, assuming you can change one constraint of the problem?

Question 2:

You have a pile of coins. One of them is counterfeit, and its weight is different from the other coins. All of the other coins weigh the same. You are given a balance. How would you find the counterfeit coin?

Question 3:

There are three buckets with a label each: oranges, apples, oranges + apples. The labels are guaranteed to be wrong, make minimum draws from the buckets and find out which bucket contains what?

Question 4:

Two airline companies, Kingfisher and Jet airways want to do a merger. Design a database migration scheme, so that no inconsistencies and redundancies. Assume suitable data and brief on the problems you might face.

Question 5:

Come up with an architecture for determining how many users are currently playing a game online. How do you scale it up and out? Where are the points of failure? Scale to 10M. Scale to 100M. Scale to 1B.

Question 6:

With two probes, determine the [clockwise/counterclockwise] direction that a plate with 2 different colors (split along a diameter) is spinning.