Citadel Coding Interview Questions
These are the 54 Citadel coding interview questions AceOffer has reconstructed from candidate reports, sorted by how often each was reported. Merge K Sorted Feeds (Cumulative Price Variant) leads at 6×. Every question shows when it was last reported; the catalog refreshes monthly.
Part of the full catalog of Citadel interview questions, covering every round.
Key facts
- •54 distinct coding questions indexed
- •104 candidate reports across those questions
- •Most reported: Merge K Sorted Feeds (Cumulative Price Variant) — 6× (last seen September 2026)
- •Refreshed monthly · last updated October 2026
Which Citadel rounds ask coding questions?
Citadel draws coding questions from the rounds below. Counts are distinct questions, not times asked.
Most reported: Merge K Sorted Feeds (Cumulative Price Variant) — 6×
Most reported: NXT OA Multiple-Choice Topics (OS, Systems, Complexity) — 6×
Most reported: Order Book / Matching Engine Implementation — 4×
Which Citadel coding questions get asked most?
Sorted by candidate-report frequency. Click any question for the full breakdown.
| Question | Round | Reported | Last seen |
|---|---|---|---|
| Merge K Sorted Feeds (Cumulative Price Variant)Free You are given K data feeds. Each feed is a list of events sorted by timestamp, and each event is (timestamp, price_delta).… | Phone Screen | 6× | September 2026 |
| NXT OA Multiple-Choice Topics (OS, Systems, Complexity)Free The Citadel NXT online assessment (90 to 120 minutes) starts with multiple-choice questions, 8, 14, about 15 or a dozen or so… | OA | 6× | September 2026 |
| Earliest Free Meeting Slot from Event StringsFree You are given a list of n event strings, events, and an integer k, the length of a meeting in minutes. Each… | OA | 5× | August 2025 |
| Count (Distinct) Palindromic Substrings (LC 647 variant)Free A palindrome is a string that reads the same forwards and backwards, such as 121 or tacocat. A substring is a contiguous… | OA | 5× | June 2026 |
| Why Citadel / Why Finance (Behavioral) Why finance? Why Citadel? Behavioral questions asked at the start of Citadel phone screens, before the coding problem. What the reports describe:… | Phone Screen | 4× | December 2025 |
| Order Book / Matching Engine Implementation Build an order book in a live coding round. Four reports describe it: two as 45-minute rounds (one of them on CoderPad),… | Coding | 4× | July 2026 |
| LRU Cache (with LFU / Custom Eviction Follow-ups) Three first-round interviews asked for an LRU cache (two are described as phone screens, the third as a first round). One of… | Phone Screen | 3× | January 2026 |
| Topological Sort of Task Dependencies Given a map tasks where each key is a task name and its value is the list of tasks it depends on,… | Phone Screen | 3× | August 2026 |
| Buy and Sell Stock: I → II → k Transactions (DP) You are given prices, where prices[i] is the price of one stock on day i. You may hold at most one share… | Coding | 3× | May 2026 |
| Best Substring by Prefix/Suffix Match ScoreFree Given three strings, text, prefixString and suffixString, score the substrings of text: - prefixScore: the length of the longest substring of text… | OA | 3× | March 2025 |
| Tree Diameter Endpoints via Double BFS (LC 1245-style)Free Online assessment, problem 3 of 3. Citadel's 90-minute HackerRank assessment (three coding problems) ends with a tree problem. Three candidates reported it:… | OA | 3× | June 2026 |
| Min Changes to Make Password a K-Periodic PalindromeFree Two problems in 75 minutes, online assessment. This is one of them. You are given a string currentPassword and an integer k.… | OA | 3× | September 2025 |
| Token Expiry and Reset Simulation (LC 1797-style)Free Authentication tokens. After a user logs in they receive a token. The token expires once the system's time limit, expiryLimit, has passed;… | OA | 3× | June 2026 |
| Trapping Rain Water (LC 42) Citadel asked LeetCode 42, Trapping Rain Water, in phone screens. The reports give only the problem number, so the statement below is… | Phone Screen | 2× | April 2025 |
| Closest Pair of Points in 2D (Divide and Conquer) You are given N points in a plane. Find the distance between the two closest points. Start with the straightforward answer, then… | Phone Screen | 2× | August 2026 |
| Circular Buffer in C++, Then Single-Producer Multi-Consumer Two first-round interviews at Citadel Securities asked for a ring (circular) buffer: the first required the coding in C++, the second was… | Phone Screen | 2× | March 2026 |
| Order Book: Exchange BBO and NBBO Design an order book that holds order records, each with an exchange, a price, a quantity and a side (bid or ask),… | Phone Screen | 2× | September 2026 |
| Time-Based Key-Value Store (set/get by timestamp) Implement a store with two functions: - set(key, val, timestamp) records val for key at that time. - get(key, timestamp) returns the… | Coding | 2× | February 2026 |
| Best Downward Path Sum in a TreeFree Given a tree with n nodes, rooted at node 0 (nodes are numbered 0 to n-1), where values[i] is the value of… | OA | 2× | April 2025 |
| Optimise root_node() C++ from O(N²) to O(N)Free You are given a C++ function, root_node, that the task describes as correct but slow. Refactor it and speed it up: work… | OA | 2× | September 2026 |
| Prefixes Extendable to Exactly K '10' SubsequencesFree Given a binary string sequence and an integer k, you may append any number of characters, each '0' or '1', to the… | OA | 2× | September 2025 |
| Friend Recommendation by Most Common FriendsFree There are n users indexed 0 to n - 1, and m friendships given as a 2D array friendships, where each entry… | OA | 2× | April 2025 |
| Count Process Schedules with No Consecutive RepeatsFree Assign processes to time intervals so that no process runs in two consecutive intervals, and count the valid assignments. The fuller of… | OA | 2× | December 2025 |
| Max Gross Value of a Four-Way Array SplitFree Given an array arr of n integers, a triplet of 1-based indices i1, i2, i3 with 1 ≤ i1 ≤ i2 ≤… | OA | 2× | September 2025 |
| Largest Hackathon Team Under Skill-Rank ConstraintsFree There are n developers, and the skill level of the i-th developer is i, for 1 ≤ i ≤ n. The task… | OA | 2× | October 2025 |
| Trade Reconciliation: Find Unmatched Buy/Sell Trades You are given a pile of trade data. Return the trades that cannot be paired: buys and sells that do not match… | Phone Screen | 1× | September 2026 |
| Implement a BST from Scratch in C++ (insert/search/delete) Implement a binary search tree class from scratch with three operations: insert, search and delete. The round was in C++; this page… | Phone Screen | 1× | January 2026 |
| Game of Life: In-Place, Sparse and Infinite Board The second of two Citadel Securities phone screens opened with LeetCode 289, Game of Life, and then kept raising the stakes. The… | Phone Screen | 1× | March 2026 |
| Shortest Path with Fuel Tank and Gas Stations A road network is an undirected graph. Every road (edge) has cost 1, and driving one road uses 1 unit of fuel.… | Phone Screen | 1× | January 2025 |
| Median of Two Sorted Arrays (LC 4) Median of Two Sorted Arrays (LeetCode 4). What the report says One report: the first phone screen after an HR call, with… | Phone Screen | 1× | January 2025 |
| Sliding Window Top-K Elements Given an array and a window size w, slide the window across the array one element at a time. For each window,… | Phone Screen | 1× | June 2026 |
| Intersection of Multiple Arrays (LC 2248) Citadel asked LeetCode 2248, Intersection of Multiple Arrays, in a phone screen. The report gives only the problem number, so the statement… | Phone Screen | 1× | July 2025 |
| SRE Python Screening: Five Short Functions Five short Python functions, given one after another in the first round (one hour, on CoderPad). Each tab is one of them.… | Phone Screen | 1× | August 2026 |
| RPN Expression Evaluator with User-defined Operators You are given an arithmetic expression in Reverse Polish Notation as a list of string tokens, e.g. ["1", "3", "+", "2", ""],… | Phone Screen | 1× | March 2025 |
| External Merge Sort with a Heap Citadel Securities phone screen. One candidate for a C++ software engineering role reports that the phone screen was "an external merge sort… | Phone Screen | 1× | July 2026 |
| Alien Dictionary (LC 269) Asked in a summer intern interview (the report doesn't name the round), after about twenty minutes of behavioral questions. The report gives… | Phone Screen | 1× | December 2025 |
| Knight Paths on a Phone Keypad (1 to 9 in <=10 Moves) On a 9-key phone keypad, a chess knight moves in L-shapes: two keys in one direction, then one key at a right… | Phone Screen | 1× | January 2026 |
| Find All Elements Equal to K in a Sorted Array Find all elements equal to K in a sorted array. The round builds this up in steps: first the plain range, then… | Phone Screen | 1× | June 2026 |
| N-ary Tree: Sum Values, Then Link Leaves in O(1) Space A Citadel technical phone screen (one hour of coding, for a Data Storage / Infra-related role) asked three tree questions of rising… | Phone Screen | 1× | June 2026 |
| Min Subarray from Each Index Covering All Talents Phone interview: the second of two online technical rounds. You are given an array v of length n, where v[i] is the… | Phone Screen | 1× | May 2025 |
| Number of Islands (LC 200 → 694 → 305) A 45-minute Citadel phone screen for a 2026 summer SWE intern role spent about 30 minutes on one grid problem and its… | Phone Screen | 1× | December 2025 |
| Reconstruct Itinerary Without Fixed Start (Eulerian Path) Like LeetCode 332, Reconstruct Itinerary, but no starting city is given. You are given a list of one-way tickets from -> to.… | Phone Screen | 1× | September 2025 |
| React Trade Event Feed Dashboard with Pause/Resume Build a trade event feed dashboard in React: a live feed grouped by trade type, summary statistics over the last 30 seconds,… | Coding | 1× | July 2026 |
| Statistics from a 256-Bucket Frequency Array You are given an array of length 256 whose entries are frequencies. For example, [2, 5, 8] means 0 appears 2 times,… | Coding | 1× | July 2026 |
| Deduplicating File System: writefile / readfile Files with the same content waste storage. For example, /a/b/c.txt and /a/b/e.txt both hold the string "hello". How would you optimise the… | Coding | 1× | September 2026 |
| Find Servers That Handled Most Number of Requests (LC 1606) Asked in the first of three 45-minute virtual onsite rounds, after a self-introduction and a discussion of an internship project and its… | Coding | 1× | April 2025 |
| Implement tac (Reverse cat) with Progressive Optimisation Implement tac, the reverse of cat: given a file, print its lines starting from the last line and ending with the first.… | Coding | 1× | November 2025 |
| Thread-Safe Key Counter, Then Cross-Process Write a counter that returns how many times a key has been called so far. Then make it thread safe, and finally… | Coding | 1× | May 2026 |
| Search Suggestion System (Trie, LC 1268) Search Suggestions with a Trie (close to LeetCode 1268). What the report says One report, from the first of two first-round virtual… | Coding | 1× | August 2026 |
| Find Median from Data Stream (LC 295) Find Median from Data Stream (LeetCode 295). What the report says One report, from a Citadel Securities software engineering intern process: after… | Coding | 1× | August 2025 |
| Largest Team with a Core Employee OverlapFree Form the largest team of employees such that at least one member, the core employee, has office hours that overlap the hours… | OA | 1× | December 2025 |
| Source Key-Value Override in First-Seen Order (NXT OA)Free Online assessment (NXT Engineering Test): the first of two coding problems. You are given sources, a 2D array of strings. Each row… | OA | 1× | October 2025 |
| All Achievable MEX Values After Increment OperationsFree There are n memory blocks, and the size of the i-th block is memoryBlocks[i] (0 ≤ i < n). The operation: select… | OA | 1× | October 2025 |
| Min Cost Image Processing with Daily Discount (OA)Free Reported by a Citadel Software Engineer intern (US) candidate as the second of two problems in a 75-minute round. Input: an integer… | OA | 1× | July 2025 |
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