Oracle Appplication Developer interview questions

They asked only puzzles, not a single question from DS, Algo, C, C++, Java or DBMS.

At first they asked explain one of my projects. Then they asked questions on OS, DBMS, etc. and 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 which possess the numbers that are multiples of i. Eg- first person will open all the doors(as i=1 is the factor of all the numbers, 2nd person will close door no 4, 6, 8... etc). At the end of 100th person how many doors will remain open and closed?? 2. Try to Sort a stack in O(n) time.