Citadel Coding Interview Questions
25 Citadel coding interview problems with full optimal solutions — 8 easy, 12 medium, 5 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Citadel interviewer values, and a FAQ section.
Showing 3 problems of 25
- #1easyvery frequently asked
1. Two Sum
Citadel uses Two Sum as a calibration problem in SWE phone screens — they want to see if you immediately reach for a hash map rather than nested loops. At Citadel's latency targets, the difference between O(n) and O(n²) at 10M ticks/sec is not academic.
- #66easyoccasionally asked
66. Plus One
Plus One probes your carry-propagation instinct — the same mechanics underpin arbitrary-precision arithmetic libraries used in financial systems for exact decimal calculations. Citadel interviewers watch whether you handle the all-nines edge case (where a new leading digit is needed) cleanly and without special-casing.
- #121easyvery frequently asked
121. Best Time to Buy and Sell Stock
This problem is more at home at Citadel than anywhere else — it's literally what the firm does. Interviewers use it to see whether you recognize the 'running minimum' pattern and can articulate why a single-pass scan is sufficient. Expect follow-ups on multiple transactions and transaction costs.