Design and implement a minimal banking service with the following capabilities: 1) Create bank accounts with unique IDs and an initial balance; support deposit and withdrawal operations while preventing overdrafts. 2) Maintain per-account transaction history. Each record should include a timestamp, operation type (deposit, withdrawal, transfer_pending, transfer_in, transfer_out), amount, and resulting balance. Provide APIs to fetch the full history and the most recent N entries. 3) Implement inter-account transfers that execute only if the recipient explicitly accepts them. A transfer starts in PENDING and does not change balances until accepted; if rejected or expired, it is canceled. Ensure idempotent accept/reject handling, validate sufficient funds at acceptance time, and make operations safe under concurrent requests. Expose clear method signatures (e.g., createAccount, deposit, withdraw, createTransfer, acceptTransfer, rejectTransfer, getHistory). Describe chosen data structures, outline time/space complexity for core operations, and discuss edge cases (duplicate requests, invalid accounts, negative amounts, and clock/timestamp considerations).

This question evaluates a candidate's skills in system design, data modeling, transactional integrity, concurrency control, idempotency, API design, and algorithmic complexity for implementing a minimal banking service.

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This is a Medium difficulty Coding & Algorithms question, commonly asked during Take-home Project rounds at HubSpot.

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Design and implement a bank account system | HubSpot Interview Question

Design and implement a minimal banking service with the following capabilities:

Create bank accounts with unique IDs and an initial balance; support deposit and withdrawal operations while preventing overdrafts.
Maintain per-account transaction history. Each record should include a timestamp, operation type (deposit, withdrawal, transfer_pending, transfer_in, transfer_out), amount, and resulting balance. Provide APIs to fetch the full history and the most recent N entries.
Implement inter-account transfers that execute only if the recipient explicitly accepts them. A transfer starts in PENDING and does not change balances until accepted; if rejected or expired, it is canceled. Ensure idempotent accept/reject handling, validate sufficient funds at acceptance time, and make operations safe under concurrent requests. Expose clear method signatures (e.g., createAccount, deposit, withdraw, createTransfer, acceptTransfer, rejectTransfer, getHistory). Describe chosen data structures, outline time/space complexity for core operations, and discuss edge cases (duplicate requests, invalid accounts, negative amounts, and clock/timestamp considerations).

Design and implement a minimal banking service with the following capabilities:

Create bank accounts with unique IDs and an initial balance; support deposit and withdrawal operations while preventing overdrafts.
Maintain per-account transaction history. Each record should include a timestamp, operation type (deposit, withdrawal, transfer_pending, transfer_in, transfer_out), amount, and resulting balance. Provide APIs to fetch the full history and the most recent N entries.
Implement inter-account transfers that execute only if the recipient explicitly accepts them. A transfer starts in PENDING and does not change balances until accepted; if rejected or expired, it is canceled. Ensure idempotent accept/reject handling, validate sufficient funds at acceptance time, and make operations safe under concurrent requests. Expose clear method signatures (e.g., createAccount, deposit, withdraw, createTransfer, acceptTransfer, rejectTransfer, getHistory). Describe chosen data structures, outline time/space complexity for core operations, and discuss edge cases (duplicate requests, invalid accounts, negative amounts, and clock/timestamp considerations).

Design and implement a bank account system

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Design and implement a bank account system

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