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DoorDash·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

DoorDash coding round for a software engineering role. The problem was themed around their core product which was a nice touch, but the actual challenge had enough moving parts to keep you honest.

Questions Asked (1)

Q1

You're consuming a stream of order events from an upstream API. Each order has an accept time and a fulfill time, making it 'active' between those two points. Pay is $0.30 per minute, multiplied by how many orders overlap in that same minute. Build a system that mocks the upstream API and computes total dasher pay for an arbitrary set of overlapping orders.

Algorithms & Data StructuresAPI & IntegrationsSystem Design
Author's notes

The multiplicity part is what gets you.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design a mock API that generates order events with accept and fulfill times. For computing pay, use a sweep-line algorithm: create events for each order start and end, sort them, and sweep through time to maintain the count of active orders, accumulating pay per minute based on the overlap count.

Pro tip: Discuss how you would handle large-scale data and real-time updates, and mention that the sweep-line approach can be extended to process streaming events incrementally, which is crucial for production systems.

1. Clarify Requirements and Edge Cases

Ask questions to understand the expected input format, whether orders can have zero duration, if times are inclusive/exclusive, and the scale of data. Clarify that pay is computed per minute and that overlapping orders multiply the rate.

2. Design the Mock API

Outline a simple mock API that returns a list of orders with accept and fulfill timestamps. Consider using a fixed dataset or generating random orders for testing, and ensure it mimics the upstream API's behavior.

3. Choose the Algorithm

Select a sweep-line algorithm: convert each order into two events (start and end), sort events by time, and sweep through while maintaining a count of active orders. At each minute, add pay = 0.30 * active_count.

4. Implement and Test

Write code to process the events, compute total pay, and test with various scenarios including no overlap, full overlap, and partial overlap. Validate against a brute-force method for small inputs.

5. Discuss Scalability and Optimizations

Explain how the solution can handle large datasets by processing events in a stream, using efficient data structures, and potentially parallelizing. Mention time complexity O(n log n) due to sorting.

Key Points to Mention

  • Sweep-line algorithm for efficient overlap computation
  • Event-based processing: start and end events for each order
  • Time complexity analysis: O(n log n) for sorting, O(n) for sweep
  • Handling edge cases: zero-duration orders, orders ending at the same time as others start
  • Mock API design: simplicity, reproducibility, and alignment with real API
  • Scalability considerations: streaming, large data, and potential optimizations

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