Upstart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Upstart software engineer interview with a simulation-style coding problem that looks manageable until you actually try to implement it. The queue and revenue tracking logic trips you up if you're not careful about the 'pay once' constraint.

Questions Asked (1)

You're given a buffet with a fixed seating capacity, a list of customer prices, and an event stream where each customer ID alternates between arriving and leaving. Implement a function that returns total revenue, where a customer only pays the first time they're seated, never on return visits.

Algorithms & Data StructuresSystem Design

Author's notes

The event encoding is what gets you first.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose an efficient solution using a hash set to track seated customers and a counter for current occupancy. Simulate the event stream, charging only on first arrival when capacity allows, and return the total revenue.

Pro tip: Mention that you would handle invalid inputs gracefully and discuss how the solution scales with large event streams, showing awareness of real-world data issues.

1. Clarify Requirements and Edge Cases

Ask about input formats, capacity limits, price ranges, and what happens if a customer arrives when the buffet is full or leaves without having arrived. Confirm that revenue is only from first-time seated customers.

2. Design Data Structures

Use a hash set to track customers who have already paid (seated at least once) and a counter for current occupancy. This allows O(1) checks for first-time arrivals and capacity.

3. Simulate the Event Stream

Iterate through events in order. For an arrival, if the customer is not in the paid set and capacity allows, add to paid set, increment occupancy, and add price to revenue. For a departure, decrement occupancy if the customer is currently seated.

4. Handle Edge Cases and Validate

Consider scenarios like duplicate arrivals, departures without arrival, capacity full, and empty events. Ensure the solution handles them correctly and discuss potential optimizations.

Key Points to Mention

Use a hash set for O(1) lookup to track first-time customers.
Maintain a current occupancy counter to enforce seating capacity.
Only charge on the first arrival when capacity allows; ignore return visits.
Process events sequentially to respect the order of arrivals and departures.
Discuss time and space complexity: O(n) time, O(n) space for n events.
Mention edge cases: full capacity, invalid departures, and duplicate arrivals.

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