Yes thats right, you can come up with any solution. But make sure O(1) is satisfied (both in time and space complexity).

Anyways I had failed to give proper answer and I was rejected in the interview. This was the last question of my last round!

Spoiler: Here is one such solution using a simple but very clever trick: http://www.geeksforgeeks.org/design-a-stack-that-supports-ge...