Citadel interview notes, page 4 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

May 2026

Citadel software engineer interview with a graph/topology problem. Pretty standard for this kind of role but the problem has enough edge cases to trip you up if you're not careful.

Given a sorted list of words from an alien language that uses the English alphabet, derive the ordering of letters in that language. Return the letters sorted by the alien language's rules, an empty string if no valid ordering exists, or any valid ordering if multiple exist.

“Classic topological sort problem once you see it, but getting there takes a minute.”

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Citadel·Software Engineer·Technical Phone Screen

May 2026

Citadel quant engineer interview with a pretty gnarly algorithmic problem about maintaining a running median under memory constraints. Not a lot of context given about the round or outcome but the problem itself was memorable enough to post about.

How would you implement a running (cumulative) median when memory is limited?

“The classic two-heap approach came to mind immediately but they pushed back on memory usage pretty fast.”

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Citadel·Software Engineer·Technical Phone Screen

May 2026

Citadel quant engineer interview with a heavy focus on probabilistic simulation. The core question was about generating normal samples from uniform random numbers, and they wanted actual code plus a real explanation of why the math works.

You have a uniform random number generator. How would you use it to produce samples from a standard normal distribution? Walk through multiple approaches and implement one.

“This one has layers.”

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Citadel·Software Engineer·Online Assessment (OA)

May 2026

Citadel SWE assessment, four algorithmic problems, all pretty gnarly. The kind of session where you finish and genuinely aren't sure if you passed or just produced a lot of confident-looking wrong code.

Given a binary string and an integer k, count how many non-empty prefixes of the string can be extended (by appending '0's and '1's) to produce a string with exactly k subsequences equal to "10".
Given an integer array, choose three cut indices to split it into four contiguous segments (possibly empty) and maximize the value computed as sum(s1) - sum(s2) + sum(s3) - sum(s4). Cuts can coincide and can sit at the array boundaries.
There are n developers each with a unique skill level from 1 to n. Each developer i has constraints on how many team members can have skill below them and how many can have skill above them. Find the largest subset where every member's constraints are satisfied.
You have an array of memory block values. You can increment any element as long as it stays below n-1. After any sequence of operations, find all distinct MEX values that are achievable, returned in ascending order.

“This one wrecked me a bit.”

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Citadel·Software Engineer·Technical Phone Screen

May 2026

Citadel software engineer interview with a classic stock trading problem. Pretty straightforward as far as these things go, though the pressure of the setting makes even simple problems feel heavier than they are.

Given an array of daily stock prices, find the maximum profit from at most one buy followed by one sell on a later day. Return 0 if no profit is possible.

“Knew the answer pretty quickly but still fumbled the first explanation.”

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Citadel·Software Engineer·Onsite - Behavioral / Leadership

May 2026

Behavioral round at Citadel for a software engineering role. Pretty much every question was scenario-based and they wanted real examples with numbers attached, not vague stories about teamwork.

Tell me about a project where you owned the design from start to finish.
Describe a time you made a significant technical trade-off and had to explain your reasoning to people without a technical background.
Walk me through a time you identified and resolved a production issue.
Tell me about a mentorship experience, either as the mentor or the mentee.
Give me an example of a project where you had to work closely with product managers, designers, data teams, or operations to get something shipped.

“This one I actually felt decent about.”

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Citadel·Data Scientist·Technical Phone Screen

May 2026

Citadel data scientist interview that went deep on probability and statistics, the kind of session where you realize pretty fast whether your math fundamentals are actually solid or just surface-level. Four questions, all technical, no behavioral fluff.

If three random variables X, Y, Z all have the same pairwise correlation ρ, what is the minimum value ρ can take?
In a simple linear regression of Y on X, if you know R² and the slope β(y|x), can you derive the slope β(x|y) from the reverse regression of X on Y?
X and Y are independent Uniform(0,1). Compute the covariance between max(X, Y) and min(X, Y).
Given a monotone function Y = g(X), how do you derive the pdf of X if you know the pdf of Y?

“This one tripped me up more than I expected.”

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Citadel·Software Engineer·Technical Phone Screen

May 2026

Citadel SWE interview with a classic stock problem. Nothing too wild but the pressure of the setting made me second-guess myself more than I should have.

Given an array of daily stock prices, find the maximum profit from a single buy-sell transaction, or return 0 if no profit is achievable.

“Knew this one cold but still fumbled the explanation a bit.”

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Citadel·Software Engineer·Technical Phone Screen

May 2026

Citadel software engineer interview with a classic Game of Life problem. The in-place constraint is what makes it interesting and where most people, including me, need a minute to think.

Given an m x n grid where each cell is either alive or dead, implement Conway's Game of Life and compute the next board state in-place using O(1) extra space.

“The rules themselves are easy to recite.”

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Citadel·Data Scientist·Technical Phone Screen

May 2026

Citadel Data Scientist technical screen, pretty focused on Python internals. One question, but they really wanted you to go deep on it rather than just sketch something out.

What does lazy evaluation mean in Python, and can you write a generator using yield that produces an infinite Fibonacci sequence?

“The infinite part is what tripped me up mentally at first.”

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