Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Stripe coding interview, looks like a follow-up to a classic multi-currency balance problem. The twist here is adding a constraint that no balance can go negative, which sounds minor until you're mid-implementation.

Questions Asked (1)

You have a system tracking per-account, per-currency balances. Given a list of transactions, process them in order but reject any transaction that would cause a balance to drop below zero. Return the final state of all balances.

Algorithms & Data StructuresData Modeling

Author's notes

The base version of this problem is pretty standard, but the rejection logic tripped me up a bit.

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AI HintsAI Generated

Suggested Approach

Clarify the data model and transaction semantics, then propose a single-pass solution using a hash map keyed by (account, currency) to track balances. For each transaction, check if the source balance would go negative; if not, apply the debit and credit atomically, otherwise reject it. Finally, return the map of balances.

Pro tip: Mention that in a real system, you'd need to handle concurrency and idempotency, but for this problem, a simple in-memory map suffices. Also, discuss how you'd extend it to support multiple currencies per account without mixing them.

1. Clarify requirements and assumptions

Ask about transaction types (e.g., transfers, deposits, withdrawals), whether amounts are always positive, and if accounts can have multiple currencies. Confirm that rejection means the transaction is skipped entirely and does not partially apply.

2. Design the data structure

Use a hash map with a composite key (account ID, currency) mapping to the current balance. This allows O(1) lookups and updates, and naturally separates balances per currency.

3. Process transactions sequentially

Iterate through the list in order. For each transaction, check if the source account has sufficient balance in the given currency. If yes, update both source and destination balances; if no, reject the transaction (skip it).

4. Handle edge cases

Consider self-transfers (same account and currency), zero-amount transactions, and non-existent accounts (treat as zero balance). Ensure that rejection does not affect any balances.

5. Return final balances

After processing all transactions, return the map of balances, possibly filtering out zero balances or including them as needed. Discuss time and space complexity: O(n) time, O(m) space where m is number of unique (account, currency) pairs.

Key Points to Mention

Use a hash map with composite key (account, currency) for O(1) balance lookups.
Process transactions in order and reject atomically if insufficient funds.
Handle multiple currencies per account by keeping separate balances.
Consider edge cases: self-transfers, zero amounts, missing accounts.
Time complexity O(n) and space complexity O(m) where m is number of unique account-currency pairs.
Mention potential real-world concerns like concurrency and idempotency, but keep focus on the algorithmic solution.

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