Citadel Interview Experiences

15 results

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  A Deep Dive on the Classic Orderbook ProblemCitadel · Software Engineer · Intern · Technical Screen · Sep 2026

  This was my second round for the Citadel SWE intern process. The technical part was the classic orderbook problem. The question was the classic orderbook problem. Problem description: Design and implement an order book data structure that maintains order records containing the exchange name, price, quantity, and side (bid or ask). It needs to support three core APIs: add(exchange, price, quantity…

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  File Deduplication, Size Checks, and Reference CountsCitadel · Software Engineer · Intern · Technical Screen · Sep 2026

  My first Citadel SWE interview. The interviewer was on the hedge fund side. I gave a brief introduction. Before I had a chance to tell the behavioral stories I'd prepared, he went straight into technical questions. The interview was basically 100% coding and system design. Problem: Suppose a file system contains files with identical contents. For example, /a/b/c.txt and /a/b/e.txt both contain th…

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  Merging K Timestamped Price Feeds into a Running Price in Technical Round 3Citadel · Software Engineer · Intern · Technical Screen · Sep 2026

  Citadel SWE intern technical interview. The interviewer was from Citadel Securities. Problem description: You are given K data streams (feeds), and each feed is already sorted by timestamp in ascending order. Each event has two fields: a timestamp and a price delta. Requirement: merge the events from all the feeds in time order, and after processing each event, output the current cumulative absol…

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  A Course Schedule Variant That Didn't Quite RunCitadel · Software Engineer · Intern · Technical Screen · Aug 2026

  This was for a 2027 summer internship, interviewing in the summer before my junior year. The first 15 minutes were self-introduction plus resume questions. The resume question was pretty vague — the interviewer only asked one question, but it actually bundled two things together: my most technically challenging experience, and whether I had a feature that directly helped with product commercializ…

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  A Maddeningly Vague Data Sampling QuestionCitadel · Software Engineer · Technical Screen · Aug 2026

  Citadel is out of their minds. The first half was fine — a normal self-introduction, project walkthrough, basic behavioral questions. Then the coding part started and that's where the craziness began: The interviewer was mumbling and couldn't speak clearly, and gave me a purely verbal problem: "we have millions of data points that need to be labeled," and told me to find the useful data. I kept f…

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  Optimizing a Correct C++ Function to Run Under 100 MicrosecondsCitadel · Software Engineer · Online Assessment · Aug 2026

  The assessment provided C++ source code for a root_node function. Its logic was correct, but it ran extremely slowly. The task was to refactor and optimize it so that it would finish in strictly less than 100 microseconds on large inputs. The original code contained nested loops and operations such as std::find_if and std::advance, producing O(N²) complexity. I needed to reduce that to O(N). It a…

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  Spreadsheet Coding Round, Rejected for Weak System DesignCitadel · Software Engineer · Technical Screen · Aug 2026

  Rejected

  Sharing my recent first technical round for an experienced-hire role on Treasury Engineering at the Citadel hedge fund. It was in Python and lasted about an hour. For the first 15 minutes or so we talked about my background. They asked whether I prefer research or engineering, what I like most and least about my current job from a technical point of view, and whether I take part in production sup…

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  Onsite Design Round Interrupted Nonstop by a Confused InterviewerCitadel · Software Engineer · Onsite · Jul 2026

  I just finished interviewing for Citadel Securities' C++ SWE position, and the whole experience was very negative, so I'm posting here to vent. The phone screen was an external merge sort with a heap. The interviewer was decent, and the overall experience was fine. After that, they quickly scheduled the onsite and told me there'd be five rounds total: design + 2h coding + 1h code review + 1h proj…

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  Two First Rounds and Progressive Matrix QuestionsCitadel · Software Engineer · Onsite · May 2026

  Rejected

  I asked a lot of people, and they all said they had two first-round interviews. I received both as well, but I did not make it to the second round. The first one: I was asked only one question: share some especially difficult problems you worked on recently and explain them. For the coding portion, a prefix tree or trie was enough to solve it. The question was very similar to a search suggestion…

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  Temperature Models, Interpolation, and Streaming RegressionCitadel · Data Scientist · Technical Screen · Apr 2026

  First question Background You are given daily temperature data for P towns and daily temperature data for New York City over the same N days. Write functions to answer several questions. Part One — five questions Q1: Which place, either one of the towns or New York City, has the greatest temperature variation, measured by standard deviation? Q2: When Town2's temperature is between 90 and 100 degr…

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  2027 OA, Two Problems in 75 MinutesCitadel · Software Engineer · Online Assessment · Mar 2026

  There were two problems, with 75 minutes in total. First problem: Given an initial work schedule for n days, an employee earns a fixed amount for each working day. If the previous day was also a working day, the employee earns an additional bonus. The employee may change at most k days off into working days, but may not change a working day into a day off. The goal is to choose which days off to…

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  A Late, Awkward Interviewer and One Linear-Fit QuestionCitadel · Data Scientist · Technical Screen · Jan 2026

  The technical interview was supposed to be 30 minutes, but the interviewer showed up 5 minutes late. Fine, whatever. Then he introduced himself, and the whole thing was really awkward — he stumbled over his words the entire time, and all he actually said was his name and his job title. Then he just froze there, like maybe he was supposed to say something about the company, but he said nothing at…

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  HackerRank OA, Two Coding Problems in 75 MinutesCitadel · Software Engineer · Online Assessment · Dec 2025

  I did this OA on the HackerRank platform. Two questions, 75 minutes. Q2: You're given n employees' office hours as intervals [startTime[i], endTime[i]]. You need to build a team that includes at least one "core employee" whose working hours overlap with every other member's working hours. Find the maximum number of employees the team can include. For example: Input: startTime = [2, 5, 6, 8], endT…

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  OA Questions Matched a 1point3acres CompilationCitadel · Software Engineer · Online Assessment · Sep 2025

  I was tested on the last two of these. My solutions passed all the test cases. All of the questions below are from 1point3acres. Prefix Count of "10" Given a binary string sequence of length n, you can perform the following operation any number of times: append a '0' or '1' to the end of the sequence. After updating the sequence (i.e. appending characters to the end of the original sequence), if…

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  Two OA Problems I Couldn't Solve OptimallyCitadel · Software Engineer · Online Assessment · Jul 2025

  I wrote up my Citadel interview experience around this time last year too, and it was pretty easy back then. This time I got the OA invite and just went ahead and did it, and I ended up not finding the optimal solution for either problem, haha. If anyone has ideas, let's discuss. Problem 1 Given an array arr of n integers, for a triplet of 1-based indices i[1], i[2], i[3] (1 ≤ i[1] ≤ i[2] ≤ i[3]…