cPacket Networks Software Engineer interview questions

The interviewer firstly talks about the resume at first. Then he asks me some basic questions. For example, describe a virtual function and difference between mutex and semaphore. Then he gives an algorithm question: given N numbers ranging from 1 to N without duplicate, how to find a missing number if a number is missing. This is just for warm up. Then he asks what if two numbers are missing. I gave three solutions but none of them is expected by the interviewer.