Simple Bank System
A medium-tier problem at 61% community acceptance, tagged with Array, Hash Table, Design. Reported in interviews at Capital One and 5 others.
Simple Bank System shows up in design rounds at Capital One, Coinbase, Dropbox, and Airbnb. It looks straightforward on the surface: implement a bank with deposit, withdraw, and transfer operations. But the real test is how you handle edge cases without over-engineering. The 60% acceptance rate signals that candidates often miss subtle constraints or design their solution inefficiently. This is exactly the problem where you either nail the logic in the live OA or you hit a wall halfway through and watch time drain. StealthCoder surfaces a clean working solution the second you blank on the state management.
Companies that ask "Simple Bank System"
Simple Bank System is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe trick isn't the operations themselves, it's tracking account state correctly and validating each action without letting bugs hide in conditions. Most candidates write the core logic, then get tangled in whether to check balance before or after decrementing, whether transfers fail atomically, and how to handle the account ID map efficiently. Hash Table stores accounts by ID; Array logic is minimal. The real pitfall: writing checks that look right but don't cover all edge cases, or implementing withdraw and transfer logic separately when they're almost identical. Common failure mode is returning a misleading result code when operations partially succeed. When you're live and realize your validation logic is fragile, StealthCoder pulls a bulletproof solution that handles deposits, withdrawals, and transfers with the right state semantics in seconds.
Pattern tags
You know the problem.
Make sure you actually pass it.
Simple Bank System recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Simple Bank System interview FAQ
How much harder is this than it looks?+
The operations are trivial, but the design demands you think about invariants: can you withdraw more than your balance, what happens if a transfer fails mid-operation, which return values mean what. 60% acceptance means a lot of candidates code sloppily and fail edge cases. It's a design problem masquerading as implementation.
Do I need to handle concurrent access?+
The problem statement doesn't specify. Assume single-threaded unless told otherwise. If asked about concurrency in follow-up, mention locks or atomic operations, but don't bake it in initially. Most solutions don't require it.
Should I use Array or Hash Table for accounts?+
Hash Table by account ID is the right call. You're doing O(1) lookups by ID, not iterating. Array doesn't add value here and makes the code messier. The topics list includes both, but Hash Table is the natural fit.
What's the most common mistake candidates make?+
Not thinking through transfer semantics. Most write separate withdraw and deposit logic, then transfer calls both. If the deposit fails, you've already withdrawn. Good solutions treat transfer as atomic or clearly document rollback behavior.
Is this still asked at FAANG?+
Yes. Capital One, Coinbase, Dropbox, and Airbnb all report it. It's a warm-up design problem that separates candidates who think carefully about state from those who code reflexively. 60% pass rate suggests it filters well.
Want the actual problem statement? View "Simple Bank System" on LeetCode →