It was part of the on-campus placement process. There was an aptitude test, which included Quant, LR, DI, Verbal, and CS subjects (OS, DS, Algorithms, DBMS). Then there were 2 to 3 technical rounds, and at last, the HR round. I was rejected at the first technical round.

At first, they asked me to explain one of my projects. Then, they asked questions on OS, DBMS, etc. Lastly, there was a puzzle and a coding question, as follows:

1. There are 100 doors (initially closed), numbered 1 to 100, and 100 persons. Starting from i=1, the i-th person will open (if closed) or close (if open) the doors whose numbers are multiples of i. For example, the first person will open all the doors (as i=1 is a factor of all numbers). The second person will close door numbers 4, 6, 8, etc. At the end of the 100th person, how many doors will remain open and closed?

2. Try to sort a stack in O(n) time.