ML Coding: Matrix Prefix Products, Backprop and Hillis-Steele Scan

CodingOpenAILast reported August 2026Medium Frequency

Reported: 6× across candidate reports
First seen: April 2026
Last reported: August 2026
Reported outcome: mixed

Problem Overview

OpenAI coding question, reconstructed from 6 candidate reports (April 2026 to August 2026). Candidates reported 3 follow-ups. The write-up includes a Python solution that passes 8 test cases.

3 candidate-reported follow-ups — the probes candidates report being asked, with the trigger for each
The full 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. Follow-ups candidates reported, not invented ones.

Codex Fact-Checked

Technical claims, formulas, and scale numbers are reviewed against primary sources.

Interviewer Follow-ups

The follow-ups candidates reported, with the trigger that prompts each one — plus the mistakes that sink people.