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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
- Questions
- 546
- Companies
- 11
- Topics
- 8
- Languages
Showing 1141–1170 of 4616 results
- CitadelMediumSoftware EngineerCompute BBO and NBBO from order data
This question evaluates a candidate's skill in designing efficient data structures and algorithms for aggregating and querying order-book data, including computing best bid/ask prices and summing quantities at those prices.
Coding & AlgorithmsTechnical Screen - CitadelMediumData ScientistCompute P(third Ace | Ace in first two)
This question evaluates understanding of conditional probability, dependence versus independence, symmetry arguments, and combinatorial reasoning in sampling without replacement.
Statistics & MathTechnical Screen - CitadelHardSoftware EngineerCompute Statistics from a Frequency Array
Compute count, minimum, maximum, mean, median, mode, and population standard deviation directly from a 256-bin frequency array. Use weighted moments and cumulative ranks without expanding samples, define empty-data behavior and ties, and address overflow and numerical precision.
Statistics & MathTechnical Screen - CitadelMediumData ScientistCompute variance of trading profits
This interview question evaluates statistical assumptions, formulas, estimation strategy, uncertainty, edge cases, and interpretation in a realistic interview setting. A strong answer for Compute variance of trading profits states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
Statistics & MathTechnical Screen - CitadelEasyData ScientistDecide Whether an LSTM Is Appropriate for Sequence Data
Decide whether an LSTM is appropriate for sequential data by defining the sequence unit, prediction horizon, masking, and time-safe validation. Compare the model with lag-based and simpler baselines while considering data size, overfitting, latency, and maintainability.
Machine LearningTechnical Screen - CitadelMediumData ScientistDerive Coefficient and Covariance in Regression Analysis
Evaluates statistics fundamentals across equicorrelation constraints, reverse regression slopes, covariance of uniform order statistics, and change of variables. Strong answers use positive semidefinite matrices, simple-regression identities, order-statistic expectations, and Jacobian transformations.
Statistics & MathTechnical Screen - CitadelHardData ScientistDesign Framework for Robust House-Price Prediction Model
Evaluates robust house-price prediction modeling across diagnostics, features, scale, and validation. Strong answers cover linear regression diagnostics, Cook's distance, leverage, Random Forest complexity controls and variable importance, housing-market feature engineering, leakage prevention, large-scale training, and time-geography validation.
Machine LearningTechnical Screen - CitadelMediumSoftware EngineerDesign Inventory Availability and Order Placement
Design e-commerce inventory availability and order reservation without overselling under concurrent requests. Define on-hand, reserved, and available invariants; atomic multi-item placement; idempotent retries; expiration; and optimistic versus pessimistic control for hot SKUs.
System DesignTechnical Screen - CitadelMediumSoftware EngineerDesign a Thread-Safe Shared Counter
This question evaluates concurrency and synchronization competencies, including atomicity, race conditions, concurrent data structures, and cross-process coordination for maintaining exact per-key counts.
Software Engineering FundamentalsTechnical Screen - 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 - 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 - 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 - 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 - 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 - 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 - 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