3 Stripe OA Problems That Test More Than Algorithms

I wouldn't treat these as three more algorithm boxes to check.

The useful thread is state: parsing commands cleanly, keeping different aggregates separate, and knowing exactly when an event changes the result.

1. Request Routing System — medium — Full-time OA

This is a command processor before it's a distance problem. Keep the pipeline boring: parse, validate, read state, choose a datacenter, then mutate its load. The bugs are usually duplicate registration, invalid coordinates, capacity, health, and distance/name tie-breaking—not the Haversine formula itself.

https://www.fastprep.io/problems/stripe-request-routing-system

2. Merchant Fraud Risk Scoring — medium — New Grad / Intern OA

Three passes touch the same transactions for different reasons. Use separate maps for merchant score, `(customer, merchant)` counts, and `(customer, merchant, hour)` counts. Trying to compress every rule into one clever pass makes it much harder to verify which update happened first.

https://www.fastprep.io/problems/stripe-merchant-fraud-risk-scoring

3. Incident Monitor — hard — Full-time OA

This is the one I'd save for last. You need a 30-second window and active alert state per `(merchant, status code)`. A threshold staying true should not emit another trigger; a later merchant event can resolve it; and the final events still need deterministic ordering.

https://www.fastprep.io/problems/stripe-incident-monitor

I'd solve them in that order and write down the persistent state before coding each one. Then I'd add one test for a tie, one for an exact threshold, and one for a trigger-resolve-trigger cycle. If those tests are hard to express, the state model probably isn't clean yet.

The FastPrep catalog currently shows these three last seen from Aug 3 through Aug 21, with two to four catalogued sightings each. They are unofficial practice formulations tied to catalogued reports, not official Stripe questions or a prediction.

If you had 90 minutes, would you spend it on the routing command processor or the incident alert lifecycle?