Design a Set of Stacks (Capacity Limit)
Implement a `SetOfStacks` data structure where a new stack is created once the previous one exceeds a threshold capacity. Implement `push` and `pop`.
Why Interviewers Ask This
Stripe engineers value systems that handle data integrity under pressure. This question tests your ability to model real-world constraints where a single buffer cannot hold infinite items. They evaluate your grasp of dynamic memory management, edge case handling like popping from an empty set, and your capacity to design modular components that scale efficiently without complex dependencies.
How to Answer This Question
1. Clarify Requirements: Ask about the specific capacity limit, whether pop should return the last element of the most recent stack, and how to handle popping from an empty SetOfStacks. 2. Define Data Structures: Propose using a list or array of individual stack objects to represent the sub-stacks. 3. Implement Push Logic: Describe checking if the current top stack is full; if so, create a new stack and push there. 4. Implement Pop Logic: Explain removing from the current top stack, and crucially, deleting the top stack if it becomes empty after removal. 5. Analyze Complexity: State that push is O(1) amortized and pop is O(1), with space complexity proportional to total elements. 6. Edge Cases: Mention handling null inputs and ensuring no index out-of-bounds errors occur during transitions between stacks.
Key Points to Cover
- Explicitly define the transition logic when a sub-stack hits capacity
- Address the edge case of deleting an empty sub-stack during pop
- Confirm O(1) time complexity for both primary operations
- Demonstrate awareness of Stripe's focus on robust error handling
- Propose a clean separation between the container list and individual stack logic
Sample Answer
To solve this, I first visualize the problem as a collection of fixed-size containers. I would implement a class containing a list of stack instances. For the push operation, I check if the current last stack has reached…
Common Mistakes to Avoid
- Failing to remove empty stacks from the list, leading to memory leaks or incorrect state
- Ignoring the scenario where popping from a single-element stack leaves the set empty
- Implementing a single flat array instead of distinct stack objects, complicating boundary checks
- Not clarifying the behavior of pop when the entire structure is initially empty
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