[leeter]

I don't recall the exact question, but I recall they were only looking for the answer that used bit manipulation. Which was not necessarily intuitive in the context.

[SilasX]

I know a common one is how to switch the values of two variables without using a temporary third variable. One way (method 2 here [1]) is to use repeated bitwise XORs.

Does that sound like it could be the one?

[1] http://www.geeksforgeeks.org/swap-two-numbers-without-using-...

[ekoontz]

Hm, perhaps counting number of bits set to 1 in a multi-byte array?

[leeter]

no, bit wise is logical there. I'm thinking it was duplicates or primes or some such.