First Unique Number: Earliest ID Seen Exactly Once in a Stream

Also reported as LeetCode 1429 · getEarliestSingleVisit · First Unique IP

Phone ScreenUberLast reported September 2026High Frequency
Reported
10× across candidate reports
First seen
October 2025
Last reported
September 2026
Reported outcome
pass

Problem Overview

Uber phone-screen question, reconstructed from 10 candidate reports (October 2025 to September 2026). Candidates reported 5 follow-ups. The write-up includes a Python solution that passes 9 test cases.

  • 5 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.
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