Google conducts interviews that are different from other American and European companies (Google Interview: What to Expect). The best way for a prospective employee to prepare is to become familiar with the interview format before sitting down in the interviewee's seat.
The interview day lasted from 10am to 4pm and consisted of 5 interviews, each lasting 45 minutes. Each interview was in-depth and demanding, not like other kinds of interview where the employer is more interested in knowing what you have done in the past. Google are interested in learning about how you approach problem-solving so the interview day focused on a number of problems. The Google interview is more a discussion between colleagues than a formal interviewer/interviewee exchange.
There were two interviews in the morning and three in the afternoon. Each interview was algorithmically focused and these problem areas discussed were:
Space and time considerations were emphasized and there was a focus on discussing the design choices, with the interviewer suggesting possible issues given the Google run-time environment. The web-page above states that you will be expected to discuss the technical details of your past designs and implementations. This was not the case at my interview which focused on solutions to the problems.
Google provided lunch, accompanied by a non-interviewing Google engineer, and this is your chance to find out more about the company, how it works day-to-day, and what life is like in that part of Switzerland. Zurich has one of the highest standards of living in Europe and English is widely spoken in the city.
Google, Zurich are a major recruiter of engineers throughout Europe. If you submit your resume, please be patient as they receives 1,000s of resumes a week and it can take time to organise a face-to-face interview as five engineers have to make time for it.
Google is famous for encouraging its engineers to work 20% of their time on a project that interests them and many of Google's recent services, such as AdSense and GMail, have come from this programme.
You can best prepare by quizzing those that have attended a Google engineer interview, remain calm, treat the interview as a discussion and relate your answers to work you have done in the past. But most of all, enjoy it. It's a great experience. Good luck with your Google interview.