Airbnb Software Engineer Interview Experience — Four-Level Banking System OA, Stuck on Level 3 at 440 Points

Level 1: Basic banking operations. Needs to support:

create_account(timestamp, account_id)
deposit(timestamp, account_id, amount)
transfer(timestamp, source_account_id, target_account_id, amount)

Level 2: Top Spenders

Requirement:

rank accounts based on the total value of outgoing transactions

Adds something like:

top_spenders(timestamp, n)

Level 3: Scheduled Payments. The system needs to support:

• schedule payment
• accept payment

The biggest trap here is the timestamp execution order. It's not just "store a record when schedule_payment is called" and that's it. Every time an operation happens after that, you may first have to process the scheduled payments that should have executed before that timestamp.

Level 4: Merge Accounts. The last level is merge accounts. The requirement is roughly:

merging accounts while retaining both accounts' balance and transaction histories

I didn't finish the third part, only got 440 points. I'm guessing I failed.