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Stripe·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Stripe coding screen for a software engineering role. One problem, but it had enough moving parts to keep me busy for the whole session.

Questions Asked (1)

Q1

You're processing a stream of bank transactions in timestamp order. Each transaction has an account name, timestamp, currency, and amount (positive for credits, negative for debits). There's also a special platform account: whenever a non-platform account's balance would drop below zero, the platform automatically lends just enough to bring it back to zero. Track the maximum total amount ever borrowed from the platform across all accounts at any single point in time, and return that value along with the final balance for every account. Do it in O(N) time.

Algorithms & Data StructuresSystem Design
Author's notes

The running balance part was fine, I got that pretty fast.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose a single-pass O(N) solution using a hash map to track each account's balance and a running total of outstanding loans. Simulate transactions in timestamp order, applying the platform loan rule when a balance would go negative, and update the maximum borrowed amount.

Pro tip: Emphasize that the platform account itself is not subject to the loan rule, and that the maximum borrowed is tracked as a running sum of all outstanding loans, not per-account. This shows attention to detail and avoids a common pitfall.

1. Clarify requirements and edge cases

Ask about input format, timestamp ordering guarantees, currency handling, and whether the platform account can have a negative balance. Confirm that the loan is triggered only when a non-platform account's balance would drop below zero.

2. Design data structures

Use a hash map to store the current balance for each account, and maintain a running total of outstanding loans (total borrowed) and the maximum of this total seen so far.

3. Process transactions in order

For each transaction, update the account's balance. If the account is not the platform and the new balance would be negative, calculate the loan amount needed to bring it to zero, add that to the total borrowed, and update the account balance to zero.

4. Track maximum borrowed

After each transaction, compare the current total borrowed with the maximum seen so far and update the maximum if needed. This captures the peak at any point in time.

5. Return results

After processing all transactions, return the maximum total borrowed and the final balance for every account (including the platform account, which may have a negative balance representing total loans outstanding).

Key Points to Mention

  • Single-pass O(N) time complexity with O(A) space for A accounts.
  • Handling of the platform account: it is exempt from the loan rule and its balance reflects total loans given.
  • The loan amount is exactly the deficit to bring the balance to zero, not a fixed amount.
  • Maximum borrowed is a running total across all accounts, not per-account.
  • Edge cases: transactions that exactly zero out a balance, multiple loans to the same account, and the platform account receiving credits (repayments).
  • Currency: assume all amounts are in the same currency or clarify if conversion is needed.

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