Quant interview preparation

Questions

100

Traced to a firm

53

Firms

15

Updated

September 2026

Asked at

All firmsOld Mission Capital12Tower Research Capital10Jump Trading7Akuna Capital5Citadel4DED.E. Shaw3Jane Street3ACAQR Capital Management2DRW2Millennium Management2Schonfeld2SCSquarepoint Capital2Susquehanna International Group2Belvedere Trading1Optiver1

Topic

All topicsProbability10Coins, cards and games6Expected value8Statistics11Market making15Estimation and mental maths4Stochastic processes4Regression5Machine learning6Time series6Programming10Options and derivatives8Fit and motivation7

Level

AnyCoreIntermediateHard

Type

AnyBrainteaserTechnicalCaseMarket viewFit

1. 076You have K sorted arrays on disk, too large to load at once. How do you merge them into one sorted output?ProgrammingIntermediatetechnicalCitadelEquity Capital Markets · New York · 2026

   Say this

   K-way merge with a min heap of size K. Push the first element of each array into the heap, repeatedly pop the minimum and write it out, then push the next element from whichever array the minimum came from. Time is N log K, memory is O(K) plus your buffers.

   Then walk it

   1. The heap holds one candidate per array, each entry tagged with which array it came from and the index within it. Pop the smallest, emit it, and refill from that same array.
   2. Complexity: N total elements, each pushed and popped once, each operation log K. So N log K, which beats concatenate-and-sort at N log N whenever K is much smaller than N.
   3. The disk part is the real content of the question. You do not read element by element, you read blocks. Keep a buffer per array, say a few megabytes each, refill it when it drains, and write the output through a large buffer too. The heap operations are free compared with I/O, so the design goal is sequential reads and few of them.
   4. If K is very large, K times the buffer size exceeds memory, and then you merge in passes: merge groups of, say, 100 files at a time, then merge the results. That is exactly how external merge sort works, and total I/O is N times the number of passes.
   5. Practical notes I would raise: use a tournament tree or a loser tree instead of a binary heap if you want fewer comparisons per element, handle the tie-breaking rule explicitly if stability matters, and if this is a real system, check whether the operating system's readahead is already doing your buffering for you before you build it yourself.

   Where candidates lose it

   Answering merge them pairwise, which is K times N in the worst case, or ignoring the on-disk part entirely. The interviewer put the data on disk deliberately, so talk about block-sized buffered reads and what happens when K is too large to buffer. State the N log K complexity explicitly.

   Expect next

   • What if K is a million?
   • How large would you make the buffers, and why?
   • How would you parallelise it?

   Reported by candidates at Citadel (Equity Capital Markets, New York, 2026). Source: Wall Street Oasis.

2. 098How would you describe the internet to someone who has just time travelled from thirty years ago?Fit and motivationIntermediatetechnicalCitadelQuantitative Trading · Sydney · 2025

   Say this

   Anchor it in something they already have, then build outward. Thirty years ago they had a telephone network and a public library, so I would say: imagine the phone system, but instead of connecting two people it connects every machine, and instead of voices it carries copies of anything written down.

   Then walk it

   1. Start from their reference frame. Someone from the mid-nineties knows telephones, fax machines, libraries, post and television. Every explanation should be a modification of one of those, not a new concept from scratch.
   2. Then the one structural idea that makes everything else follow: the cost of copying and sending information fell to approximately zero, and nobody owns the middle of the network. From that single fact you can derive email, search, shopping, streaming and social media without explaining any of them separately.
   3. Give one concrete, checkable example. The entire contents of that library, searchable in a quarter of a second from a device in your pocket, and it is free because someone sells advertising next to the answer.
   4. Then say what they would find strangest, because that is the interesting part of the answer. Not the technology but the consequences: that attention became the scarce resource, that distance stopped mattering for information but not for goods, and that privacy became something you trade rather than something you have.
   5. Check understanding rather than lecturing. I would ask them what they want to do with it and explain that, because the tailored answer lands and the general one does not. This question is a communication test, so the move that scores is structure plus adapting to the listener, not encyclopaedic completeness.

   Where candidates lose it

   Diving into TCP/IP, packets and DNS. The question tests whether you can teach, not whether you know how it works. Anchor in their existing mental model, lead with one organising idea, and use a concrete example. Rambling without structure is the common failure, so state your three-part plan before you start.

   Expect next

   • Now explain machine learning to the same person.
   • What would they find most surprising about the consequences?
   • Do it in thirty seconds instead.

   Reported by candidates at Citadel (Quantitative Trading, Sydney, 2025). Source: Wall Street Oasis.

Firm tags come from public, anonymous candidate reports on Wall Street Oasis: strong signal, not sworn testimony. Firms are named as the places a question was reported, not as partners of Fin Maverick. Answers are written for this page to show how to think out loud; they are not scripts to recite.