Citadel Interview Questions — Page 11

Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Citadel SWE interview with a linked list reversal question. Pretty standard algorithmic stuff but they wanted both iterative and recursive solutions, plus edge case handling, so it wasn't just 'write the loop and move on'.

Given the head of a singly linked list, reverse it and return the new head. Implement both an iterative and a recursive solution, and handle edge cases like an empty list or a single node.

“The iterative part was fine, three pointers, walk the list, done.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Citadel coding interview, one meaty algorithmic problem that's basically a twist on a classic heap question. The added wrinkle of collapsing same-timestamp records before emitting made it more interesting than I expected.

Given K sorted lists of timestamped records, merge them into a single sorted stream. The catch: any records sharing the same timestamp across all lists must be combined into one record (e.g. by summing their values) before being emitted. What's your approach and complexity?

“I recognized the min-heap skeleton pretty fast, but the same-timestamp collapsing part tripped me up for a minute.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Citadel's C++ conceptual round for a Software Engineer role, no coding at all, just deep technical discussion. They really want you to know your modern C++ cold, not just the basics.

Walk me through the core OOP principles in C++ and how they interact, including the different inheritance access levels, multiple inheritance, and the diamond problem.
Explain move semantics and rvalue references in C++11. How does perfect forwarding work and when would you use it?
What are the differences between unique_ptr, shared_ptr, and weak_ptr? When would you prefer one over another?
Why does a base class with any virtual function need a virtual destructor? What actually happens if you skip it?
How does SFINAE work and how does std::enable_if let you constrain templates? How do C++20 concepts change this?
Explain RAII and the rule of 0, 3, and 5. How do these relate to exception safety?
What are the use cases for std::variant and std::optional compared to more traditional C++ approaches?
Compare static polymorphism via templates with dynamic polymorphism via virtual functions. What are the tradeoffs?

“This one sprawled.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Quant Engineer interview at Citadel that went deep into factor selection methodology. The IC/ICIR question was basically the whole conversation and they clearly wanted more than textbook definitions.

How do you use IC and ICIR to evaluate and select alpha factors, covering cross-sectional vs. time-series perspectives, horizon choice, IC decay, sector and regime robustness, ICIR thresholds, multicollinearity among high-IC factors, and how these metrics feed into portfolio construction?

“This was a single question that expanded into basically a 30-minute interrogation.”

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Citadel·Data Scientist·Technical Phone Screen

Jun 2026

Citadel data scientist interview, technical screen where they handed me a machine learning pipeline task and told me no autocomplete. The whole thing was about whether you actually know the steps cold, not just that you can Google them.

Build a complete linear regression pipeline from scratch in Python, no code completion allowed. The pipeline should handle train/validation splitting with a fixed seed, missing value imputation, one-hot encoding of categoricals, numeric scaling, OLS model training, and evaluation via RMSE and R-squared. Make sure transformers are only fit on training data. Report the fitted coefficients and interpret the results.

“This is basically a 'do you actually know ML pipelines or do you just know how to tab-complete sklearn' question.”

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Citadel·Data Scientist·Technical Phone Screen

Jun 2026

Citadel data scientist interview, heavy on probability and math. Five questions back to back, ranging from classic textbook stuff to things that actually required you to think carefully. The Squid Game one was a fun twist on a standard problem but the optimal stopping question at the end had me second-guessing myself the whole time.

There are B sequential steps on a bridge, each with two panels where exactly one is safe. P players cross in order, choosing randomly when a step is unrevealed but using known safe panels afterward. Derive the probability that the i-th player survives, and the probability that at least one of the P players makes it across.
X and Y are independent uniform(0,1) random variables. Find the PDF of their sum S = X + Y.
You roll a fair six-sided die repeatedly until every face has appeared at least once. What is the expected number of rolls?
A point is sampled uniformly from the surface of the 3D unit sphere. What is the variance of its x-coordinate?
You observe n draws from uniform(0,1) one at a time and must keep exactly k of them, deciding immediately and irrevocably after each draw. What is the optimal strategy and how do you compute the expected total value?

“The Squid Game framing made it feel approachable at first, which was a trap in itself because I started thinking about it narratively instead of mathematically.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Citadel software engineer interview that was heavier on implementation detail than I expected. The problem looked like a fun game exercise on the surface but the encode/decode part is where they actually separated candidates.

Implement a working 2048 game from scratch, including board state representation, all four directional moves with proper merge logic, and random tile spawning.
Design encode and decode functions that pack the entire 2048 board state into a single 64-bit integer and recover it losslessly.

“The move logic tripped me up more than I'd like to admit.”

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Citadel·Data Scientist·Technical Phone Screen

Jun 2026

Citadel Data Scientist interview with a pretty focused algorithmic problem. The question had a lot of layers to it and I wasn't fully prepared for how deep they wanted to go on the follow-ups.

Given a sorted integer array and a target sum, find all unique pairs of values that add up to the target. Use two pointers for O(n) time and handle duplicates so you don't return the same pair twice.
How would you modify the solution to return indices instead of values, and what does that change about the approach?
How would you adapt this approach if the array arrives as a stream and can't be fully loaded into memory?
Walk through the time and space complexity of your solution.

“The core part I got through fine.”

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Citadel·Software Engineer·Technical Phone Screen

Jun 2026

Citadel software engineer interview with a combinatorics/DP problem dressed up in a Grace Hopper theme. Pretty focused, just the one algorithmic question from what I remember.

Given n processes and m time intervals, count the number of ways to assign exactly one process to each interval such that no process appears in two consecutive intervals. Return the result modulo 10^9+7.

“The Grace Hopper framing threw me for a second, felt like a trick.”

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Citadel·Software Engineer·Onsite - System Design / Architecture

Jun 2026

Citadel software engineer interview that went deep on systems and concurrency pretty fast. The round was focused on a single design problem but it branched out into a lot of follow-ups I wasn't fully ready for.

Design a round-robin task scheduler that supports adding and removing tasks at runtime, returns the next task on each tick, and guarantees no task is starved. Discuss your data structure choices, thread-safety considerations, and the time complexity of each operation.

“Started with a deque and felt pretty good about it until the thread-safety part came up.”

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