Citadel Software Engineer Coding & Algorithms Interview Questions
Practice the exact questions companies are asking right now.
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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 ...
Simulate 2048 and pack board into uint64
Problem: 2048 move simulation + board compression You are implementing part of the 2048 game on a fixed 4×4 grid. Part A — Simulate a move Given a 4×4...
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...
Design dynamic weighted random sampling with updates
Problem: Support weighted random sampling with insert/delete Design a data structure that maintains a dynamic set of items, each with a positive integ...
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...
Compute maximum later-earlier difference
You are given an integer array output (length n ≥ 1). Define the gain of choosing indices i < j as output[j] - output[i]. Return the maximum gain over...
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...
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...
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...
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 [...
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...
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 ...
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...
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...
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 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...
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 ...
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 ...