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 8 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.
- #20easyfrequently asked
20. Valid Parentheses
Citadel asks Valid Parentheses to test stack discipline. In financial systems, bracket-matching logic underpins expression parsers for formula engines, query validators, and order-instruction parsing — getting the stack mechanics exactly right under all edge cases matters.
- #21easyfrequently asked
21. Merge Two Sorted Lists
Merging two sorted lists is the inner loop of merge sort and a primitive in time-series data fusion. At Citadel, this maps directly to merging two ordered streams of market events from separate exchanges. Interviewers watch whether you use a dummy head node and whether you handle leftover tails cleanly.
- #53easyfrequently asked
53. Maximum Subarray
Kadane's algorithm is a staple in quantitative finance: finding the highest-return contiguous window in a P&L series, identifying the best momentum run in a price series, or detecting the worst drawdown period (negate the array). Citadel interviewers expect you to name the algorithm, prove it greedy, and know the divide-and-conquer alternative.
- #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.
- #70easyfrequently asked
70. Climbing Stairs
Climbing Stairs is Citadel's entry point for dynamic programming. The interviewer is checking whether you recognize the Fibonacci recurrence, state the subproblem clearly, and then optimize space from O(n) to O(1). Expect the question to pivot toward probability: 'What if each step choice is made with probability p?'
- #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.
- #206easyfrequently asked
206. Reverse Linked List
Citadel uses Reverse Linked List to probe pointer fluency. Linked-list structures appear in lock-free queues, custom memory allocators, and order-book implementations. Getting pointer reassignment exactly right — and knowing when to use iterative versus recursive — demonstrates the low-level discipline Citadel expects.