I was trained by fb for interviewing, the phone screen, sw part. You start with a simple enough problem. See how fast the candidate can come up with a solution. Then complicate the problem to find more stuff. Like, what if you have unlimited memory? What if you have very little memory? etc. You are supposed to look for signals. Can the candidate improve the solution based on some random constraints/hints? Is the candidate good enough with the language they picked (eg, append vs extend in python, or why i copied S into s). We were not looking for tricks and the entire idea that big companies are looking for tricks in their interviews was spread by either people that never been in such an interview or were bad enough to be rejected.