I had to take an online test, which was rather difficult. I barely managed to finish it within the time given.

Then, a few days later, I was scheduled for an online interview. The interviewer seemed relatively nice but a bit awkward. He asked me very difficult programming questions, things that my school did not prepare me for at all. Google is notorious for changing its questions between almost every interview. It was very difficult.

If you didn't have to worry about time efficiency at all, but need a space efficiency of O(1), how do you check if an array has duplicate numbers?