When does the grid stop? (DSA)

I was giving Microsoft's last interview round. The ratings I might have gotten before this round would either be HIRE OR STRONG_HIRE.

In the last round I got a medium question to solve after some HR type questions.

I proposed 3 solutions O n*n and O 1 O nlog and O n O n and space O n Was on the line to solve using O n and O 1, but the interviewer said you have to take care of the int over flow so all my thought processes ended for this one.

Now the solution that he was expecting and which was marked as the top solution on the leetcode was tricky. I could not think of it

At the end, the interviewer told me the solution and I just coded it up.

My question is: Even if you do 300 questions, the interviewer might ask you a 301st question. Why are tech interviews like this? One round went bad and I won't get the offer now. I'll have to start my prep again.

FML