How do you implement a queue using stacks?
This design question tests your ability to combine data structures to achieve desired behavior. It focuses on FIFO implementation using LIFO structures.
Why Interviewers Ask This
It assesses deep understanding of stack and queue mechanics. Interviewers want to see if you can translate abstract concepts into concrete implementations. The problem also highlights your ability to manage state across multiple operations efficiently.
How to Answer This Question
Propose using two stacks: one for input and one for output. Explain the push operation simply adds to the input stack. For pop, move elements to the output stack if empty, then pop. Analyze amortized time complexity. Mention alternative approaches if relevant.
Key Points to Cover
- Two-stack architecture
- Lazy transfer of elements
- Amortized O(1) complexity
- Handling empty states
Sample Answer
I will use two stacks, inputStack and outputStack. Enqueue pushes to inputStack. Dequeue checks if outputStack is empty; if so, it pops all from inputStack and pushes to outputStack, then pops from outputStack. This ensures FIFO order. Amortized cost is O(1) per operation.
Common Mistakes to Avoid
- Transferring elements on every push
- Not clearing the input stack correctly
- Returning wrong element during dequeue
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