Citadel Coding & Algorithms Interview Questions
Citadel Coding & Algorithms interview questions focus on fast, correct problem solving under time constraints and clear communication. Interviewers typically evaluate algorithmic intuition, data-structure mastery, complexity analysis, and code quality, alongside your ability to reason about trade-offs and edge cases. What’s distinctive about Citadel’s process is the emphasis on practical efficiency and collaboration: you’ll be expected not only to produce a working solution but to explain design choices, optimize for performance, and accept hints or iterate when prompted. Expect a mix of medium-to-hard algorithmic problems, occasional systems or design discussions for senior roles, and behavioral questions that probe impact and teamwork. For effective interview preparation, simulate the real loop: complete timed coding assessments, practice on collaborative platforms like CoderPad, and rehearse explaining your approach aloud. Strengthen fundamentals in arrays, hashes, trees, graphs, dynamic programming, and complexity reasoning, and polish language-specific idioms in Python, C++ or Java. Build a short checklist to validate correctness and edge cases before submitting code, and schedule mock interviews to improve pacing and communication. Consistent, focused practice and clear articulation of thought process are the most reliable ways to perform well.

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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...
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...
Implement task queue with insert, delete, execute
Problem: Task manager with insert/delete/execute-next Design a data structure to manage executable tasks. Each task has: - taskId (unique) - priority ...
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...
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 ...
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...
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...
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...
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 [...
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 ...
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...
Solve four algorithmic problems
Answer the following independent algorithmic questions: 1) Count extendable prefixes for '10' subsequences: Given a binary string s and an integer k, ...
Minimize image processing cost with discount
Question You have n images. For the i-th image, processing costs filterCost[i] per day and must run from startDay[i] to endDay[i] inclusive. Each day ...
Implement two-pointer unique-pair sum search
Given a nondecreasing integer array nums and an integer target, return all unique value pairs [a, b] with a <= b such that a + b == target. Use the tw...
Explain hash maps and solve array intersection
1) Explain the internal implementation of a hash map: underlying array/bucket layout, hash function choice, collision resolution strategies (separate ...
Maximize Stock Trading Profits Using Dynamic Programming
Scenario Evaluating dynamic-programming skills on stock-trading profits. Question Given an array of daily stock prices and an integer K, write Python ...