Implement multi-network card validator with Luhn

Q: Implement multi-network card validator with Luhn

This is a Coding & Algorithms interview question from Stripe for Software Engineer roles. View the full question and solution on PracHub.

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Question

Design and implement a payment card validation module that supports multiple networks and error scenarios. Requirements:

Luhn checksum

Implement the Luhn algorithm: starting from the rightmost non-check digit, double every second digit; if the result > 9 subtract 9; sum all digits; valid if sum % 10 == 0.

Card networks

VISA: 16 digits, prefix 4
MASTERCARD: 16 digits, prefixes 51–55
AMEX: 15 digits, prefixes 34 or 37

Part 1 — Basic VISA

Input: a 16-digit string starting with 4
Output: "VISA" if Luhn-valid; otherwise "INVALID_CHECKSUM"

Part 2 — Multi-network

Input: a 15- or 16-digit string
Output: the network name (VISA/MASTERCARD/AMEX) if prefix/length match and Luhn-valid; "INVALID_CHECKSUM" if prefix/length match but Luhn fails; otherwise "UNKNOWN_NETWORK"

Part 3 — Redacted cards

Input: a card string containing 1–5 asterisks (*) that replace digits
Task: count how many concrete completions are valid per network, considering prefix/length rules and Luhn. Output one line per network that has at least one valid completion, sorted alphabetically by network: "AMEX, <count> ", "MASTERCARD, <count> ", "VISA, <count> ".

Part 4 — Corrupted cards

Input: a card string ending with '?' indicating exactly one error was introduced in an originally valid card, where the error is either (a) one digit changed to another digit, or (b) a swap of two adjacent digits
Task: enumerate all possible original valid cards consistent with that model (respecting network rules and Luhn). Output each as "<card_number>, <NETWORK> ", sorted numerically by card number. General constraints and expectations
Treat inputs as strings; use exact output tokens.
For Parts 3 and 4, design efficient search with pruning using prefix constraints and Luhn properties; handle large search spaces.
Specify data structures, algorithms, and time/space complexity for each part.
Describe edge cases (wrong lengths, invalid prefixes, leading zeros in redactions, multiple '*' placement, duplicate reconstructions in Part

and how you avoid duplicates and ensure determinism.

Provide test strategy and sample cases for each part.

Implement multi-network card validator with Luhn

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