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Citadel interview notes, page 8 of 14

Large Enterprises

Most of these Citadel write-ups come from Software Engineers (106), then Data Scientists (33) and Machine Learning Engineers (1). The most reported rounds are the technical phone screen (111), the system design round (16), the onsite coding round (4) and the behavioral / leadership round (3).

140 interview notes · updated Oct 2026

Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel SWE interview with a coding question that was a slightly toned-down version of a known LeetCode problem. Nothing too wild, but you still need to know your data structures.
  • Solve a coding problem involving efficient data lookup and list traversal, similar in structure to a well-known LeetCode problem but slightly simplified.

“Easier than the version I'd seen before, which honestly made me second-guess myself for a second.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel software engineer interview with a tree traversal problem that sounds straightforward but has a few wrinkles worth thinking through ahead of time.
  • Given a binary tree where each node has an additional next pointer, connect all leaf nodes in DFS traversal order using those next pointers.

“My approach was to run a DFS first, collect all the leaves into an array, then loop through and wire up the next pointers.”

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Citadel·Software Engineer·Onsite - System Design / Architecture

Jun 2026
Citadel system design round for a software engineer role. The problem was around building an alerting system for application-to-exchange mappings, which sounds manageable until they start piling on the distributed systems follow-ups.
  • Design an alerting system that detects when too many applications are being mapped to a single exchange, given a high-throughput stream of registration events.
  • What data structures and algorithms would you use to efficiently track registration counts per exchange and support a sliding window rate threshold?
  • If events are processed in parallel across multiple workers and can arrive out of order or be duplicated, how do you ensure alert decisions stay correct?

“I started by nailing down what 'too many' actually means, which felt like the right move.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel software engineer round with a pair coding session. The interviewer did a quick team intro, I did my self-intro, and then we jumped straight into the problem. Nothing too wild, but order book problems have a way of looking simple until you're actually typing.
  • Design and implement an order book system.

“Spent a minute clarifying requirements before touching the keyboard, which I think saved me.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel software engineering interview with a sliding window problem that had a twist I didn't fully see coming. The K-largest-per-window variant tripped me up more than I expected for what looked like a standard windowing question.
  • Given an integer array, a window size W, and an integer K, slide a window of size W across the array one step at a time and return the K largest elements (in descending order) for each window position.

“My first instinct was to just sort each window and slice off the top K, which the interviewer immediately flagged as too slow.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel quant engineer interview that went pretty deep into model robustness. The whole conversation basically orbited around one meaty question and they really wanted to see if you understood the practical tradeoffs, not just the textbook definitions.
  • When building factor or alpha models, how do you prevent overfitting? Walk through the main approaches you'd use.

“This felt like a warmup but it definitely wasn't.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel systems design round, focused on a single infrastructure problem that went deeper than I expected. The question sounded like a quick data structure chat but turned into a full design discussion about tradeoffs I hadn't fully thought through.
  • Design a token store that supports lazy deletion: tokens have expiration times, and on lookup you determine whether a token is still valid without proactively cleaning up expired entries. How do you structure this, and how does cleanup get amortized across operations?

“I jumped straight to a hash map and felt good about it for about 30 seconds.”

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Citadel·Data Scientist·Technical Phone Screen

Jun 2026
Citadel data science interview, heavy on probability and statistics. Three questions, all math, no fluff. The kind of interview where you either know the correlation matrix constraints or you don't.
  • Given that Corr(X,Y) = 0.8 and Corr(X,Z) = 0.9, what is the full range of possible values for Corr(Y,Z)?
  • Can you construct explicit random variables X, Y, Z such that all three pairwise correlations equal -1/2?
  • Using only a Uniform(0,1) random number generator, how would you produce samples from a standard Normal distribution?

“This one took me a minute to even remember where to start.”

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Citadel·Data Scientist·Technical Phone Screen

Jun 2026
Citadel Data Scientist interview with a pretty intense stochastic processes question. The kind of thing that feels manageable until you're actually in the room trying to derive variance on the fly.
  • A stock price starts at 0 and follows a simple symmetric random walk (steps of +1 or -1). At each time step T, you buy one share if the price went up and short one share if it went down. Given that positions accumulate across time, derive the variance of your total cumulative profit at time T. Follow-ups: what's the expected profit and why, and how does this extend to continuous-time Brownian motion?

“Spent the first couple minutes just making sure I understood the position accumulation mechanic correctly, which I think was the right call because it's easy to misread.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026
Citadel software engineer interview with a coding question around stateful counters. Pretty focused, not much fluff.
  • Implement a counter function that tracks how many times each key has been invoked and returns the current count for that key.

“Seemed straightforward at first and I almost over-engineered it with a class.”

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