How do you get the minimum element from a stack in constant time?
This question tests your ability to augment data structures to support additional operations efficiently without compromising performance.
Why Interviewers Ask This
Interviewers want to see if you can optimize standard operations. It checks your knowledge of auxiliary data structures like tracking min values. The problem also reveals your ability to maintain state consistency during push and pop operations.
How to Answer This Question
Explain the standard stack operations first. Then introduce a secondary stack to store minimums. Describe how to push to both stacks when a new minimum is found. Detail the pop logic to keep both stacks synchronized. Discuss space-time trade-offs.
Key Points to Cover
- Auxiliary stack for minimums
- Synchronized push and pop
- O(1) retrieval time
- Space complexity consideration
Sample Answer
I will maintain a second stack called minStack. When pushing, if the new value is smaller than the top of minStack, push it there too. When popping, if the popped value equals the top of minStack, pop from minStack as well. The top of minStack always gives the minimum in O(1).
Common Mistakes to Avoid
- Pushing to minStack on every push
- Forgetting to pop from minStack
- Not handling duplicate minimums correctly
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