If you know something well, you can reason about it in real time. Pressure is an artifact of the interview process that is controlled primarily by the attitude

If you know something well, you can reason about it in real time. Pressure is an artifact of the interview process that is controlled primarily by the attitudes of the interviewer and interviewee regardless the subject matter. External references are useful for things you don't know cold. For something basic and fundamental to your job (eg. basic graph traversals, the suitability of a hash or a balanced BST to some lookup task, etc), you shouldn't need them; for something more arcane which is accidental rather than essential to your task, or something relatively advanced (eg. specifics of tries for incremental string operations), a competent interviewer will help you out and supply some information so that he can evaluate your ability to reason, design and code. None of these things have any bearing on whether or not it makes sense for Google to focus on knowledge of the fundamentals of computer science and systems programming in their hiring interviews.