Upstart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Upstart SWE interview with a simulation-style coding problem that took up most of the session. Pretty straightforward premise but the edge cases pile up fast if you're not careful with state tracking.

Questions Asked (1)

Design a function to compute total revenue for a buffet restaurant. The restaurant has a fixed capacity. You're given the per-customer entry fee array and a sequence of customer IDs representing entry and exit events. A customer entering while the restaurant is full is turned away with no retry. Each customer only pays on their first successful entry. Return the total revenue after processing all events.

Algorithms & Data StructuresSystem Design

Author's notes

Spent the first few minutes just re-reading the problem because the entry/exit duality in the same records array tripped me up.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the event semantics (entry/exit, capacity, payment on first successful entry) and edge cases, then propose an efficient solution using a hash set to track customers who have paid and a counter for current occupancy. Walk through a small example to validate the logic, and discuss time/space complexity.

Pro tip: Explicitly handle the case where a customer who was previously turned away tries to enter again—they should be treated as a new customer and can pay if space is available. Also, confirm whether exits can occur for customers who never entered (invalid events) and how to handle them.

1. Clarify requirements and assumptions

Ask about event format (e.g., positive ID for entry, negative for exit), capacity constraints, and whether a customer can exit without having entered. Confirm that payment occurs only on first successful entry and that turned-away customers do not retry automatically.

2. Design data structures

Use a hash set to track customers who have already paid (to avoid double charging) and an integer counter for current occupancy. Optionally, a hash set for customers currently inside to validate exits.

3. Process events sequentially

Iterate through events: for an entry, if occupancy < capacity and customer hasn't paid, add fee to revenue, mark as paid, and increment occupancy; if full, ignore. For an exit, decrement occupancy if the customer is inside.

4. Handle edge cases and validate

Consider scenarios like multiple entries by the same customer, exits without entry, capacity zero, and empty event list. Walk through a small example to ensure correctness.

5. Analyze complexity and discuss optimizations

State time complexity O(n) and space O(n) for n events. Mention that the solution is optimal for a single pass, and discuss potential variations like streaming events or concurrency if relevant.

Key Points to Mention

Use a hash set to track customers who have paid to prevent double charging.
Maintain a counter for current occupancy to enforce capacity.
Clarify event encoding (e.g., positive ID for entry, negative for exit) and handle invalid exits.
Ensure turned-away customers are not marked as paid and can pay on a later successful entry.
Walk through an example to demonstrate correctness.
State time and space complexity: O(n) time, O(n) space.

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