Citadel Interview Questions
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Design stock price time-series store and query
Problem Design a platform that stores stock prices over time and can be queried later. Core functionalities 1. Ingest price: The system receives event...
Merge K timestamped lists with timestamp coalescing
You are given k sorted lists (or linked lists) of records. Each record has: - timestamp (integer) - values (an array of integers sorted in non-decreas...
Determine order of alien alphabet
You are given a list of words sorted in lexicographic order according to an unknown ("alien") alphabet. Task Return one valid ordering of the unique c...
How do you handle conflict at work?
Describe a time you had a conflict with a teammate (e.g., disagreement on technical direction, priorities, code quality, or ownership). Please cover: ...
Compute BBO and NBBO from order data
You are given a list data of order records. Each record is a 4-tuple: ` (exchange_id, price, quantity, order_type) ` - exchange_id: string/int identif...
Compute max team size with a core interval
You are given n employees’ working-time intervals, where employee i works during the inclusive interval [startTime[i], endTime[i]]. You want to form a...
Implement array merge, round-robin scheduler, and trading simulator
You are given three independent coding prompts. For each prompt, clearly define your function/class interfaces, handle edge cases, and analyze time/sp...
Implement an in-memory order book API
Problem Implement an in-memory limit order book for a single trading symbol. You do not need to implement order matching/execution—only store and main...
Implement LRU/LFU cache with custom eviction
You are asked to implement an in-memory key-value cache with a fixed capacity. When the cache is full and a new item must be inserted, an eviction pol...
Check whether a string is a palindrome
Problem Given a string s consisting only of lowercase English letters (a-z), determine whether s is a palindrome. A palindrome reads the same forward ...
Return nodes on a tree diameter path
Problem You are given an undirected tree with n nodes labeled 0..n-1 (connected, no cycles). The diameter of a tree is the longest simple path between...
Design a low-latency trading system
System Design: Low-Latency Electronic Trading Platform (Equities) You are designing a single-region electronic trading platform (exchange/ATS-like) th...
Find earliest common meeting slot
Given K participants' calendars, each a list of busy intervals [start, end) within a working window [workStart, workEnd], and a meeting duration d min...
Diagnose outliers and influence in linear regression
OLS Diagnostics: Outliers, Leverage, Influence, and Cook's Distance Context You are fitting an ordinary least squares (OLS) linear regression with an ...
Design a token manager with lazy expiration
Problem Design a token manager that tracks authentication tokens with a fixed time-to-live (TTL). Each token is valid in the half-open time interval [...
Estimate OLS via streaming sufficient statistics
Streaming OLS and Ridge for Out-of-Core, High-Dimensional Linear Regression You need to estimate linear regression coefficients when the dataset is to...
Solve two DS&A optimization problems
Problem 1 — Maximize alternating-sum over four array partitions: Given an integer array arr[1..n] (1-based). Choose indices a, b, c with 1 ≤ a ≤ b ≤ c...
Match a string with wildcard pattern recursively
Implement a function that checks whether an input string matches a wildcard pattern. Pattern rules - ? matches exactly one character. - * matches any ...
Derive distribution of an inverse transform
Change of Variables via the Logistic Map You are given a random variable X with density f_X supported on (0, 1). Define the strictly increasing logist...
Minimize changes for k-periodic palindrome
Question Given a string currentPassword (length N) and an integer k (1 ≤ k < N, N ≤ 2·10^5, lowercase letters, N divisible by k), find the minimum num...