After submitting my resume to Facebook, I was contacted by one of their recruiters the next day. We talked on the phone about my work experience, after which they asked me to provide three dates for which they could schedule my phone coding interview.

They suggested taking at least two weeks to prepare for the interview, and they provided me with some training material. On the specified date, one of the software engineers working at Facebook called me to administer the interview. He introduced himself and asked me to tell him about the last project I worked on.

This part lasted no more than five minutes, after which he asked me two coding questions. Upon finishing both questions, the interviewer gave me an opportunity to ask any questions I might have had.

I was told I would be contacted by the recruiter to let me know about further steps. I was contacted shortly via email, but they decided not to proceed with the next interview. This came as a surprise since I answered both coding questions correctly. Unfortunately, they were not willing to provide me with feedback on the reason for my rejection.

Apart from not receiving any feedback, the interview process was a very positive experience.

Given a nested array, calculate the depth sum and provide its time and space complexity.

Input: [1, 2, [4, 5, [7], 8], 9]

Output: 1 + 2 + 2*(4 + 5 + 3*7 + 8) + 9 = 88