Circle·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Circle SWE interview, got hit with a pretty involved banking system design problem that kept layering on requirements. The scheduled payments piece was where things got tricky and I spent most of my time thinking through edge cases.

Questions Asked (1)

Design and implement a scheduled payment system for a banking application. You need a schedulePayment function that takes a timestamp, account ID, amount, and delay, then queues a payment to execute at timestamp+delay and returns a unique payment ID. You also need cancelPayment to cancel any pending payment for a given account. Critical constraint: every operation must first flush all pending payments whose execute time has passed before doing anything else. Payments execute in creation order when they share the same execute time, and a failed payment (insufficient balance) is silently dropped with no balance change and no spending recorded.

System DesignAlgorithms & Data StructuresAPI & Integrations

Author's notes

The 'flush at the start of every operation' rule is what got me.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then propose a data structure that supports efficient time-ordered execution and cancellation. Design the core operations (schedulePayment, cancelPayment, flushDuePayments) with careful attention to the flush-before-any-operation constraint, and discuss how to handle same-time ordering and failed payments. Finally, analyze time/space complexity and potential concurrency issues.

Pro tip: Emphasize that the flush-before-any-operation rule means every public method must first process due payments, which can be implemented by checking a min-heap or sorted structure. Also, note that cancellation should be lazy (mark as cancelled) to avoid O(n) removal from the heap, and failed payments are simply skipped without affecting balance or spending records.

1. Clarify requirements and constraints

Ask about expected scale, concurrency, persistence, and whether payments can be scheduled in the past. Confirm that flush must happen before any operation, including schedule and cancel.

2. Choose data structures

Use a min-heap keyed by execute time (and creation sequence for ties) to efficiently retrieve due payments. Maintain a hash map from account ID to pending payment IDs for O(1) cancellation lookup.

3. Design core operations

Implement flushDuePayments(now) that pops all payments with executeTime <= now, checks balance, and executes or drops them. schedulePayment flushes, then adds to heap and map. cancelPayment flushes, then marks payment as cancelled in map.

4. Handle edge cases and ordering

Ensure same-time payments execute in creation order by using a monotonic counter as tiebreaker in heap. Failed payments are silently dropped without balance change or spending record. Cancelled payments are skipped during flush.

5. Analyze complexity and concurrency

Discuss O(log n) for schedule and flush, O(1) for cancel (amortized). Mention thread-safety with locks or single-threaded event loop, and potential need for persistence.

Key Points to Mention

Min-heap (priority queue) for time-ordered execution with tie-breaking by creation sequence
Hash map for O(1) cancellation lookup by account ID
Lazy cancellation: mark as cancelled instead of removing from heap
Flush-before-any-operation: every public method first processes due payments
Failed payments (insufficient balance) are silently dropped with no side effects
Time complexity: O(log n) for schedule and flush, O(1) for cancel; space O(n)

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