I came onsite for what was described as an hour-long introductory and technical interview. I was greeted by the recruiter and shown around the office (FB NYC), then we chatted for 10-15 minutes. The engineer then arrived, and we started the technical part of the interview, which took 45+ minutes (we ran over).

First, the interviewer asked about Hadoop, as I had mentioned using it at my current job. I used Hadoop for machine learning tasks, so we discussed the details of the system.

The rest of the interview involved coding on a whiteboard. The questions progressively increased in difficulty.

The first question was: Given two string representations of binary numbers (e.g., "1001", "10"), write a function that adds them and returns the result as a string as well (e.g., "1011").

The next question had several parts:

(a) First, write a function to calculate the Hamming distance between two binary numbers. (b) Write a function that takes a list of binary numbers and returns the sum of the Hamming distances for each pair. (c) The solution I initially provided for (b) was O(n^2). I was then tasked with finding a more efficient solution. I struggled significantly and was eventually guided to the solution with many hints from the interviewer.

After that, I had a chance to ask questions of the interviewer, and then we were done. I don't know if I've made it to the next round or not.

(a) First, write a function to calculate the Hamming distance between two binary numbers.

(b) Write a function that takes a list of binary numbers and returns the sum of the Hamming distances for each pair.

(c) Find a solution for (b) that works in O(n) time.