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Interview Questions
Practice 9403 coding, system design and machine learning questions from 546 companies, sorted by role, topic and interview round. Every question has step-by-step hints, a code runner that supports 8 languages and an AI mentor; members also get the full solution walkthrough.
- 9403
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- 546
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- 11
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Showing 2311–2340 of 9403 results
- CitadelHardSoftware EngineerDesign a low-latency trading system
This question evaluates expertise in designing low-latency, high-determinism electronic trading systems, covering order matching, market data ingestion and publish, partitioning/sharding, persistence for auditability, idempotency, pre-trade risk controls, and operational reliability.
System DesignTechnical Screen - CitadelHardData ScientistDesign a time-series home-buy decision classifier
This question evaluates competency in ML system design for time-series decisioning, including rigorous target and horizon definition, temporal feature engineering, time-aware validation, probabilistic calibration and thresholding, leakage controls, model selection trade-offs, and deployment monitoring.
ML System DesignOnline Assessment - CitadelMediumSoftware EngineerDesign a token manager with lazy expiration
This problem tests your ability to design a token management system that supports efficient retrieval, updates, and lazy removal of expired tokens using a combination of a hash map and a priority queue.
Coding & AlgorithmsTechnical ScreenHash MapHeapTTL - CitadelMediumSoftware EngineerDesign alerting for application-to-exchange mappings
This question evaluates understanding of real-time stream processing, alerting policy design, and distributed state coordination for application-to-exchange mappings, testing competencies in scalability, correctness under partitioning and duplication, and efficient aggregation of high-volume event streams.
System DesignOnsite - CitadelMediumSoftware EngineerDesign an Authenticated Online Shopping Service
Design an authenticated online shopping service with secure signup and login, user-owned carts, product browsing, and retry-safe order creation. The solution covers password hashing, session rotation, CSRF defenses, resource-level authorization, idempotency keys, immutable order snapshots, recovery tokens, and horizontal-access tests.
System DesignOnsite - CitadelHardData ScientistDesign city home-price prediction system
This question evaluates proficiency in end-to-end machine learning system design for geospatial property price prediction, covering skills such as feature engineering, time- and spatial-aware validation, model selection, interpretability, fairness/compliance, deployment, monitoring, and error analysis.
Machine LearningTechnical Screen - CitadelMediumSoftware EngineerDesign dynamic weighted random sampling with updates
This question evaluates a candidate's ability to design and analyze dynamic data structures that support weighted random sampling with inserts and deletes, testing skills in algorithm design, complexity analysis, and randomized sampling under large numeric constraints.
Coding & AlgorithmsOnsite - CitadelMediumData ScientistDetermine When a Quadratic Has Finite Minimum
This question evaluates understanding of quadratic optimization, linear algebra, and convexity concepts—particularly properties of quadratic forms, matrix symmetry, and conditions for boundedness in real-valued unconstrained minimization—and why these properties matter for existence of minima.
Machine LearningTechnical Screen - CitadelEasyData ScientistDiagnose and Validate a Regression Model
Work through regression model validation from split design and leakage prevention to residual analysis and segment-level error checks. The solution explains MAE versus RMSE, multicollinearity, coefficient limits, and when added model complexity is worth the operational cost.
Statistics & MathTechnical Screen - CitadelHardData ScientistDiagnose outliers and influence in linear regression
This question evaluates a data scientist's competency in diagnosing outliers, high-leverage points, influential observations, and Cook's distance in ordinary least squares regression, including recognition of robust alternatives and rationale for reporting model-review decisions.
Machine LearningTechnical Screen - CitadelMediumData ScientistDiscuss PhD coursework and research impact
This question evaluates a candidate's foundations in empirical modeling, ability to learn from failed approaches, incorporation of feedback, and skill in quantifying research impact within a research or product context.
Behavioral & LeadershipTechnical Screen - CitadelHardSoftware EngineerDiscuss a Project You Are Proud Of and How You Work
Use one truthful project to explain what you are proud of, what changed in your thinking, and how you organize a normal workday. Connect personal decisions and measurable impact to later behavior while showing realistic prioritization, collaboration, focused work, and adaptability.
Behavioral & LeadershipTechnical Screen - CitadelHardSoftware EngineerDiscuss queues, NoSQL, and concurrency
This question evaluates understanding of distributed messaging systems, delivery semantics, push versus pull consumption models, NoSQL versus relational storage trade-offs, and concurrency control including deadlock concepts.
System DesignTechnical Screen - CitadelHardData ScientistEstimate OLS via streaming sufficient statistics
This question evaluates proficiency in streaming/out-of-core linear regression, including computing sufficient statistics with an intercept, assessing numerical stability of normal equations versus QR/SVD or incremental methods, incorporating ridge penalties, and designing parallel fault-tolerant computations.
Machine LearningTechnical Screen - CitadelEasyData ScientistEvaluate Alternative Data for an Investment Pitch
Evaluate an alternative dataset for an investment pitch through provenance checks, a falsifiable signal hypothesis, and point-in-time validation. Test incremental value, selection bias, revisions, robustness, and costs before presenting the thesis, risks, and invalidation criteria.
Analytics & ExperimentationTechnical Screen - CitadelHardSoftware EngineerEvery MEX Reachable With At Most One Bounded Increment
A coding problem about memory blocks with values below n: using at most one increment of a single value, never raising a value to n, list every minimum excluded value (MEX) the array can end up with. It tests careful case analysis of how one increment can open or fill a gap, handling duplicate values, and returning the sorted answer efficiently.
Coding & AlgorithmsOnline Assessment - CitadelHardSoftware EngineerEvery Reachable Priority Gap After At Most One Bounded Bump
Given n network ports each tagged with a priority code below n, find every value the lowest unused priority code could take after at most one bounded increment. Tests careful case analysis of how a single bump opens or closes a gap, and returning the answer efficiently.
Coding & AlgorithmsOnline Assessmentarrayscountingmex - CitadelMediumSoftware EngineerExplain JS types, Promises, Maps, WebSockets
Explain JS types, Promises, Maps, WebSockets evaluates requirements, assumptions, structured reasoning, trade-offs, and verification in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
Software Engineering FundamentalsTechnical Screen - CitadelMediumSoftware EngineerExplain career motivations and choices
This question evaluates self-awareness, career motivation, role-fit, decision-making, and communication skills for a Software Engineer by probing career values, educational and internship choices, and motivation for entering the finance industry.
Behavioral & LeadershipTechnical Screen - CitadelMediumData ScientistExplain factor leakage checks and IC/ICIR filtering
This question tests the conceptual understanding of signal quality metrics and the practical steps required to prevent look-ahead bias and overfitting when developing quantitative trading factors.
Machine LearningTechnical ScreenQuantitative FinanceAlpha ModelingBacktesting - CitadelMediumData ScientistExplain multicollinearity and OLS assumptions
Explain multicollinearity and OLS assumptions evaluates statistical assumptions, formulas, estimation strategy, uncertainty, edge cases, and interpretation in a realistic interview setting. A strong answer states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
Statistics & MathTechnical Screen - CitadelEasyData ScientistFind constant minimizing absolute error
This question tests the understanding of L1 loss minimization and the statistical property that the median is the optimal point estimator under absolute error loss.
Statistics & MathTechnical ScreenStatisticsOptimizationMedian - CitadelMediumSoftware EngineerFind earliest common meeting slot
This question evaluates proficiency with interval manipulation, timeline merging, and algorithmic reasoning for scheduling constraints, measuring competency in handling sorted interval lists, edge-case sanitization, and complexity analysis.
Coding & AlgorithmsTechnical Screen - CitadelMediumSoftware EngineerFind the Index Range of a Target in a Sorted Array
Implement an algorithm to find the first and last index of a target value in a sorted array in O(log n) time.
Coding & AlgorithmsTechnical Screenbinary-searcharrayscitadel - CitadelMediumSoftware EngineerFind the Shortest Target-Sum Path
This problem tests your ability to navigate binary trees using recursion or iterative traversal while maintaining state to find a path that meets a specific sum criteria with the smallest number of nodes.
Coding & AlgorithmsTechnical ScreenTreesDFSRecursion - CitadelHardSoftware EngineerFirst Free Locker Number After One Tag Bump
Given locker tags in [0, n-1], list every possible smallest missing tag after at most one capped +1 adjustment. Tests MEX reasoning, careful case analysis around duplicates, and an O(n) solution.
Coding & AlgorithmsOnline Assessmentarrayscountingcase-analysis - CitadelMediumSoftware EngineerGenerate Prefix-Based Product Suggestions
Generate up to three lexicographically smallest product suggestions for every prefix of a search word, with efficient handling once a prefix has no matches.
Coding & AlgorithmsOnsite - CitadelMediumSoftware EngineerHarbor Clash: A Two-Captain Terminal Naval Game, Setup Then Combat
Write a runnable two-player hidden-fleet game for the terminal: captains place vessels via typed commands with full validation, then alternate firing until one fleet is sunk; tests modeling, parsing, turn flow and shipping working code fast.
Software Engineering FundamentalsTechnical Screenobject-oriented-designgameinput-validation - CitadelMediumSoftware EngineerHow do you handle conflict at work?
This question evaluates interpersonal skills such as conflict resolution, communication, accountability, and leadership by asking about a concrete disagreement with a teammate in a software engineering context.
Behavioral & LeadershipOnsite - CitadelMediumData ScientistImplement Infinite Fibonacci Generator Using Lazy Evaluation
This question evaluates understanding of lazy evaluation and Python generator semantics, testing a candidate's ability to work with iterative sequence production and manage state across iterations.
Coding & AlgorithmsTechnical Screen
Citadel