How do you implement a queue using two stacks?
This question evaluates your ability to simulate data structures using other primitives, testing logical mapping skills.
Why Interviewers Ask This
Understanding the relationship between LIFO and FIFO structures demonstrates deep algorithmic insight. Financial systems often rely on stack-based parsing or queue-based task scheduling. This tests if you can optimize amortized costs.
How to Answer This Question
Explain the two-stack strategy: one for enqueueing and one for dequeueing. Describe how moving elements from the input stack to the output stack reverses their order. Analyze the amortized time complexity, noting that while a single pop might be O(n), the average is O(1).
Key Points to Cover
- Input stack for enqueues
- Output stack for dequeues
- Transfer logic on empty output
- Amortized O(1) analysis
Sample Answer
I would use one stack for push operations and another for pop operations. When popping, if the output stack is empty, I transfer all elements from the input stack to the output stack. This reversal ensures the oldest element is at the top of the output stack. The amortized cost per operation remains constant.
Common Mistakes to Avoid
- Transferring elements on every push instead of pop
- Incorrectly managing stack states during transfer
- Failing to explain amortized complexity
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