Affirm phone screen, Live Fraud Detector writeup

Affirm phone screen, Live Fraud Detector writeup

Sharing this since other people's writeups on Affirm's phone screen really helped me prep, figured I'd add to the pile. Got the Live Fraud Detector problem, 60 minutes on codepad with full screen share required.

Part 1 is debugging a DistinctPIIValuesCounter class that's supposed to track distinct PII values, address, phone, email, ssn, seen across underwriting events. The starter code was missing a check that filters out non underwriting event types before counting, so it was double counting stuff it shouldn't have touched at all. Took a few minutes to spot since the bug wasn't in the obvious counting logic, it was upstream in the filter.

Part 2 builds on that, implementing a FraudDetector that flags suspicious underwriting events and then propagates any PII seen on a flagged event forward into a suspicious set so future events with matching PII also get flagged. Easy to miss that newly flagged events need to propagate their PII forward too, people tend to assume only the events already in the suspicious set matter.

No real algorithmic complexity once you get the propagation rule, it's mostly a hashmap and hashset problem, but reading the 100 plus lines of starter code carefully ate a good chunk of the hour. Finished with about 5 minutes left, waiting to hear back now.