Citadel / Citadel Securities Online Assessment

Software Engineering — SWE / SDE / NG intern

Two coding problems drawn from a large rotating pool shared across the SWE, SDE and new-grad intern screens (often a resume-screen filter).

• Maximize the Minimum Gap in a Subsequence Coding · medium
• Most-Visited Position Across Sprints Coding · easy
• Maximize Cards Bought Within a Budget Coding · easy
• Shortest Substring to Rewrite for a Balanced Gene String Coding · medium
• Count Song Pairs Summing to a Whole Minute Coding · easy
• Minimum Knight Moves on an N-by-N Board Coding · medium
• Prime Factor Visitation Coding · medium
• Triplets Coding · medium
• Inversions Coding · medium
• Ways to Sum Coding · medium
• Do They Belong? Coding · medium
• Consecutive Numbers Sum Coding · medium
• First Lady of Software Coding · easy
• Math with Lego Blocks Coding · easy
• Process Execution Coding · medium
• Special Nodes Coding · medium
• Hacker's Team Coding · medium
• IPO Coding · hard
• Consistent Logs Coding · medium
• Distinct Bitwise-OR Goodness Coding · medium
• Minimum Operations to Finish Parallel Jobs Coding · medium
• Longest Subarray With All Elements Greater Than K Coding · easy
• Minimum Server Upgradation Time Coding · medium
• Maximum Workday Earnings Coding · medium
• Minimum Generation To Equalize Neural Layers Coding · medium
• Get the Groups Coding · easy
• Date Format Conversion Coding · easy
• Maximum Profit Buying One Share Per Day Coding · easy
• Validate Binary Tree And Serialize Coding · medium
• Count Subarrays Divisible by m Coding · medium
• Counting Inversions via Modified Merge Sort Coding · medium
• Maximum of Sliding Window Minimums Coding · easy
• Longest Subarray With Sum at Most K Coding · easy

Quant Research / Quant Trading — QR / QT

Two formats are reported for the Citadel Securities trading OA. A July 2025 QT candidate describes about 15 fill-in-the-blank probability and statistics questions (other prep sites cite 30 minutes); older HackerRank sittings (2019-2020) ran 10 probability questions in four separately timed sections, 23 or 30 minutes in total. Both formats have timed replicas, alongside the recruiter-sourced QR phone-screen brain-teasers.

Citadel Securities QT OA, 2025 format (15 fill-in / 30 min): free sample →

Full 15-question simulation (Pro) →

Classic sectioned format (10 questions, 4 timed sections): free sample →

Full classic simulation, 30 minutes (Pro) →

• Probability of no HH in N coin flips Probability · medium
• Biased-coin Bayes update Probability · medium
• Welford online mean and variance Statistics · medium
• Regression slopes and R-squared Statistics · medium
• 24 Game Coding · hard · free
• Best-of-five expected games Probability · medium
• Von Neumann fair coin from a biased coin Probability · medium · free
• Maximum drawdown of a price series Coding · medium
• Monte Carlo estimate of pi and e Probability · medium · free
• Pancake sorting Coding · medium
• Rock climber expected jumps Probability · medium
• Eight consecutive equal answers in 10 T/F Probability · medium
• Two same-colored sock pairs (4 black, 4 white) Probability · medium
• Buy/sell stock timing Coding · easy

Data Science — Datathon

A multiple-choice data-science round mixing pandas/R analysis with statistics, probability and ML theory; the 2024 version tests both Python and R rather than letting you pick one.

• Choosing an Explainable Stock-Picking Model ML theory · easy
• Probability One Sample Mean Beats Another by a Year Statistics · medium
• True Statements About Correlation and Causation Statistics · easy
• Two coins, one double-headed Probability · medium
• Normal approximation to the binomial Statistics · medium
• Poisson-pulse mod-N counter Probability · hard
• Pandas isin row filter pandas/R · easy
• ggplot2 grammar of graphics pandas/R · easy
• k-medoids, Naive-Bayes and elastic-net theory ML theory · medium