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Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Stripe coding screen, looked like a straightforward OOP extension problem but the state management piece tripped me up more than I expected.

Questions Asked (1)

Q1

You're given an existing AccountScheduler class with a constructor and an is_available method. Add an acquire method that locks an account for a given duration starting at time t, returning false if the account is already locked.

API & IntegrationsAlgorithms & Data Structures
Author's notes

Seemed simple at first, just set a value in a dict and return true or false.

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AI HintsAI Generated

Suggested Approach

First, clarify the requirements and constraints, such as whether locks can overlap, how to handle time intervals, and if concurrency is a concern. Then, design the data structure to store lock intervals, ensuring efficient overlap checks. Finally, implement the acquire method with proper validation and return values, and discuss potential optimizations.

Pro tip: Mention that you would use an interval tree or a sorted list of intervals for efficient overlap detection, and consider thread-safety if the scheduler is accessed concurrently. Also, discuss how to handle edge cases like zero-duration locks or locks that start in the past.

1. Clarify requirements

Ask questions to understand the expected behavior: Can locks overlap? What is the granularity of time? Should the method be thread-safe? What should happen if t is in the past?

2. Choose data structure

Decide on a data structure to store existing locks, such as a list of intervals, a sorted list, or an interval tree, balancing simplicity and efficiency based on expected usage.

3. Implement overlap check

Write a helper function to check if the new interval [t, t+duration) overlaps with any existing lock intervals. Consider edge cases like adjacent intervals.

4. Implement acquire method

Use the overlap check to determine if the account is available. If available, add the new interval and return true; otherwise, return false.

5. Discuss optimizations and edge cases

Mention potential improvements like using a balanced BST for O(log n) checks, handling concurrency with locks, and cleaning up expired locks.

Key Points to Mention

  • Time complexity of the overlap check and how it affects scalability
  • Handling of edge cases: zero-duration locks, locks starting in the past, adjacent intervals
  • Thread-safety and concurrency considerations if the scheduler is shared
  • Choice of data structure and its trade-offs (e.g., list vs. interval tree)
  • Integration with existing is_available method and consistency
  • Potential need for lock expiration or cleanup to avoid memory bloat

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.