Google Software Engineer - Intern interview questions

I received an email requesting a phone interview with 2 people at separate times but back to back. The email also contains two links to two google docs. This is where you type in your answers so the interviewer can see them. The types of questions they ask you is about algorithm and problem solving in general, but they go deeper and deeper once you have a solution. They'd go with the "what if now..." questions. Luckily I scored a third interview, and this was just like the other past two, over the phone and on a google doc. But I didn't get an offer after this.

this is my first phone interview, the process is very technical. it is two 45min, with 15 min rest time in the middle, and I have been asked questions about writing a tree-related algorithm, describing basic operating system ideas. I can only remember this much.