Circle·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026

Summary

Circle SWE interview, banking system design problem that kept adding levels. Level 4 introduced account merging which sounds straightforward until you're actually tracking pending delayed payments and reassignment ordering in your head.

Questions Asked (2)

Implement a mergeAccounts function that merges a source account into a target account: combine balances, combine total spending stats used by topSpenders, reassign all pending scheduled payments from source to target ordered by execution time then creation order, and delete the source account. Return false if either account doesn't exist or if source and target are the same.

Algorithms & Data StructuresSystem DesignData Modeling

Author's notes

The balance and spending sum parts were fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the data model and requirements, then outline a transactional algorithm that validates inputs, updates the target account's balances and spending stats, reassigns pending payments with proper ordering, and deletes the source account. Emphasize atomicity and edge cases to ensure data consistency.

Pro tip: Mention that the operation should be wrapped in a database transaction to prevent partial updates, and discuss how to handle concurrent merges or failures gracefully.

1. Clarify Requirements and Data Model

Ask questions to understand the account structure, balance types, spending stats format, and payment scheduling details. Confirm assumptions about atomicity and error handling.

2. Validate Inputs

Check that both source and target accounts exist and are distinct. Return false immediately if validation fails.

3. Merge Balances and Spending Stats

Add the source account's balances to the target account's balances. Combine total spending stats (e.g., sum amounts and counts) used by topSpenders.

4. Reassign Pending Scheduled Payments

Retrieve all pending payments from the source account, update their account reference to the target, and sort them by execution time then creation order. Ensure the target's payment list remains ordered.

5. Delete Source Account and Ensure Atomicity

Remove the source account from the system. Wrap all operations in a transaction to guarantee atomicity and discuss rollback strategies.

Key Points to Mention

Transactional integrity and atomicity to avoid partial updates
Data consistency and concurrency control (e.g., locking accounts during merge)
Efficient data structures for merging and sorting payments (e.g., priority queue or merge sort)
Edge cases: non-existent accounts, same account, empty payment lists, large datasets
Scalability considerations if accounts have many payments or high transaction volume
Idempotency and error handling to allow safe retries

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

Implement a getBalance function that returns the current balance of an account after first processing all pending scheduled payments with an execution time at or before the given timestamp. Return null if the account doesn't exist or has been merged away.

Algorithms & Data StructuresSystem Design

Author's notes

This one builds on earlier levels so if you've been consistent with your delayed payment logic it's mostly plumbing.

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AI HintsAI Generated

Suggested Approach

Clarify the data model and requirements first, then design an efficient solution that processes pending payments up to the timestamp and handles account existence and merge status. Discuss trade-offs between different data structures and algorithms, and consider concurrency and scalability for a system like Circle's.

Pro tip: Mention that you would process scheduled payments lazily on read to avoid unnecessary background jobs, and ensure idempotency so payments aren't double-processed if getBalance is called multiple times with the same timestamp.

1. Clarify Requirements and Assumptions

Ask about the data model: how accounts, scheduled payments, and merges are represented. Confirm whether the timestamp is inclusive, and whether processing payments should mutate state or be computed on the fly.

2. Design Data Structures

Propose storing scheduled payments in a min-heap or sorted list keyed by execution time, and maintaining a mapping from account ID to account object with balance and merge status. Consider using a balanced BST or priority queue for efficient retrieval.

3. Outline Algorithm

Describe the steps: check if account exists and is not merged; if not, return null. Otherwise, retrieve all pending payments with execution time <= timestamp, apply them to the balance, and return the updated balance. Ensure payments are processed in chronological order.

4. Analyze Complexity and Optimizations

Discuss time complexity: O(k log n) for processing k payments using a heap, or O(log n + k) with a sorted structure. Mention lazy processing to avoid scanning all payments, and caching for repeated calls.

5. Address Edge Cases and Concurrency

Cover edge cases: no pending payments, account merged, timestamp before all payments, and duplicate calls. Discuss concurrency control (e.g., locking or optimistic concurrency) to handle simultaneous getBalance and payment scheduling.

Key Points to Mention

Account existence and merge status check before processing
Efficient retrieval of pending payments using priority queue or sorted list
Inclusive timestamp comparison and chronological processing
Idempotency and lazy evaluation to avoid double-processing
Time and space complexity analysis with trade-offs
Concurrency and consistency in a distributed system

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