The HR reached out to me after the OA and scheduled a preliminary screening call.

After that was the first technical round.

There were supposed to be two more technical rounds and one final Hiring Manager round.

I wasn't able to make it past the first technical round.

In the HR round, questions included: "What made you apply?" and "Why Uber?"

In the first tech round, I was asked a binary tree question: to skew it all to the right (unbalance it). I was able to come up with a brute-force solution and explained it to the interviewer, but he asked me to develop an O(1) space solution. I believe I performed well and was able to code the solution, but I struggled to explain it correctly. I think that's where it went awry. :(