Card Validation (Luhn)

OAStripeLast reported August 2025Low Frequency
Reported
2× across candidate reports
First seen
June 2025
Last reported
August 2025
Reported outcome
unknown

Problem Overview

Implement a multi-part payment card validation system using the Luhn algorithm for Stripe. Card Networks: - VISA: 16 digits, starts with 4 - MASTERCARD: 16 digits, starts with 51–55 - AMEX: 15 digits, starts with 34 or 37 Luhn Algorithm: Starting from the rightmost digit (the check digit), move left…

  • 3 candidate-reported follow-ups — the exact probes interviewers asked, with the trigger for each
  • The rest of the problem statement — full requirements, constraints, and edge cases
  • Approach and trade-offs — what passing candidates did, and the mistakes that sink people
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Verified Source
Every question is reconstructed from multiple independent candidate reports. Verbatim follow-ups, not invented ones.
Codex Fact-Checked
Technical claims, formulas, and scale numbers are reviewed against primary sources.
Interviewer Follow-ups
The exact follow-ups reported by candidates, with the trigger that prompts each one — plus the mistakes that sink people.
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