Match payments to invoices by memo or amount

Q: Match payments to invoices by memo or amount

This Coding & Algorithms question for a Data Scientist evaluates string parsing and pattern matching, keyed and numeric record matching, date-based tie-breaking, and algorithmic complexity analysis, and is commonly asked to test handling of messy input formats and deterministic reconciliation rules.

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Q: What role is this question designed for?

This question is commonly asked for Data Scientist candidates at Google during technical interviews.

Question

You are building a small payment-to-invoice matching utility.

Data

You are given:

invoices : a list of invoice records with:
- invoice_id (string)
- amount_cents (integer)
- due_date (string, ISO-8601 date like YYYY-MM-DD )
payments : a list of payment records with:
- payment_id (string)
- amount_cents (integer)
- memo (string; may contain extra whitespace)

Assume invoice_id is unique in invoices.

Part 1 (Memo → invoice_id matching)

A payment memo may contain a standardized invoice reference in the form:

"Paying off: <INVOICE_ID>"

where <INVOICE_ID> is the invoice id you should match to.

Write a function that, for each payment, extracts the target invoice_id from the memo (if present), finds the corresponding invoice in invoices, and outputs a matched result.

Output requirement (per payment):

If a match is found: output the payment_id , extracted invoice_id , and the matched invoice details.
If no match is found (or memo cannot be parsed): output a clear "not found" result for that payment.

Handle edge cases such as extra spaces in the memo, missing/empty memo, or malformed standardized strings.

Part 2 (Hybrid matching: memo-based OR amount-based)

Extend the logic:

If memo contains the standardized prefix "Paying off:" , use Part 1 memo → invoice_id matching.
Otherwise, match by amount:
- Find invoices where invoice.amount_cents == payment.amount_cents .
- If there are multiple candidates, choose the invoice with the earliest due_date .
- If there is no candidate, output: "cannot find matching invoice for amount <amount_cents>" (or equivalent).

State the time and space complexity of your approach and justify it.

Match payments to invoices by memo or amount

Quick Overview

Data

Part 1 (Memo → invoice_id matching)

Part 2 (Hybrid matching: memo-based OR amount-based)

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