Whatsapp has a different hiring philosophy. They want you to do everything right in the interview. This basically means you have to do remember every nook and corner ins software world. They do not appreciate you figuring it out on the fly and giving an answer. There is no room for error, even on a white board, which I think is pretty stupid. It leads to rejection of perfectly good candidates so they can say "We interviewed 100 candidates but gave only 3 offers" kind of marketing. They asked me about how I would convert a 32-bit number into a string. I did the standard division by two and final string reversal algo. My algo was wrong for the case when input is zero, since I used a while loop instead of a do-while loop. In normal production environment, it is easy to catch but on white board, its not. It turned out that what the interviewer really wanted was an algo which scaled for millions of calls on server, which he didn't initially specify. This means, one has to a small table to store the strings and then do bit operations. Unfortunately, the interviewer has more context about the question than what we interviewees do. A similar example is when the interviewer asked me to calculate median delivery time of 100B messages. Discussion turned to choosing a good strategy for sampling the 100B messages. I suggested that we take traffic patterns into account and suggested we do sampling every second. He said he preferred uniform sampling (take every nth message). Again, I had the feeling that based on some internal metrics, they decided that uniform sampling is best. I tried telling him that he has more context than me but it was lost...as I am seeking a job not him. Most of the people there are over 35, mostly from Yahoo. In conclusion, whatsapp looks for a like small clique to me.