Jane Street Interview Questions — Page 4

Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview with a tree parsing and decryption problem. Fairly involved for a single question but the kind of thing Jane Street is known for throwing at you.

Given a substitution cipher key and a pre-order serialized tree string, decode the tree and return it as nested lists where each node contains its decrypted paragraph text and an ordered list of children.

“Two things happening at once here: reversing the cipher and parsing the serialized tree structure.”

View Post

Jane Street·Machine Learning Engineer·Onsite - Coding / Algorithms

Jun 2026

Coding round at Jane Street for an ML Engineer role. One problem, but it had enough edge cases to keep me busy for the whole session. The board simulation stuff felt almost like a puzzle game at first, then got annoying fast.

Simulate a 2D game board where pieces are always inserted at the bottom of a column, pushing existing pieces up. After each insertion attempt, report whether it succeeded, whether any fully-filled column is uniform in piece type, and whether any fully-filled row is uniform in piece type.

“My first instinct was to just brute-force scan the whole board after every operation and I said so out loud.”

View Post

Jane Street·Software Engineer·Onsite - System Design / Architecture

Jun 2026

Jane Street system design round focused entirely on order book data validation across multiple databases. Pretty brutal in scope, they wanted you to think through invariants, cross-db reconciliation, edge cases, and operationalization all in one go.

What invariants must always hold for orders, executions, and the resulting order book state?
How would you validate data quality within a single database, using table-level constraints and batch checks?
How do you reconcile data across multiple databases that may only be eventually consistent?
How do you handle edge cases like partial fills, cancellations, order replaces, out-of-order events, duplicates, and reconnect replays in your validation logic?
How would you operationalize this validation system, including alerts, dashboards, and backfill processes?

“This is where I started strong and then trailed off.”

View Post

Jane Street·Data Scientist·Technical Phone Screen

Jun 2026

Jane Street Data Scientist interview with a game theory problem involving a 21-sided die. The core question was deceptively clean but the optimal strategy took some real work to pin down, especially resisting the urge to just accept anything nonnegative.

Two players take turns rolling a fair 21-sided die (faces labeled -10 through 10). On each turn, the roller can accept the face value as a payment from the opponent, or reject and pass the die. Both play optimally. What is the optimal strategy for each player, and what is the expected payoff to the first player?

“My first instinct was 'accept anything >= 0' and I was pretty confident about it for an embarrassingly long time.”

View Post

Jane Street·Data Scientist·Technical Phone Screen

Jun 2026

Jane Street Data Scientist interview with a game theory brain teaser that sounds deceptively simple but turns into a full-on Nash equilibrium derivation under pressure. The interviewer keeps asking 'why' until you've either proved your answer or fallen apart.

Two players each secretly pick an integer from 1 to 100. The larger pick has 10 subtracted from it, then that result is compared to the smaller pick. Whichever value is larger wins. What is the optimal strategy, and can you prove it?
For the number-picking game: when exactly does the player who chose the larger number win, lose, or draw? Work it out as a function of the gap between the two picks.
Prove that no pure (deterministic) strategy is optimal in this game, and derive the lowest number a rational player should ever consider picking.
Among the numbers that survive the dominance elimination, is there one that beats all the others? Describe the win/loss structure within that range and find the optimal mixed strategy.
How does the optimal strategy change if the penalty subtracted from the larger pick is k instead of 10, or if the range goes up to N instead of 100?
If your opponent picks uniformly at random over 1 to 100, what is your best response and what is your expected payoff?

“This one wrecked me a little.”

View Post

Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street coding interview for a software engineer role, one question on implementing a ring buffer. Pretty focused session, no fluff.

Implement a circular (ring) buffer with full functionality. Time complexity is not a concern.

“Appreciated that they said time complexity didn't matter, but I still got a bit tangled deciding between an array-based approach and a linked list.”

View Post

Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview with a pretty gnarly combinatorics/search problem. The kind of question where the constraints look small and you think you're fine, then you realize the search space is way weirder than it looks because of rational arithmetic and arbitrary parenthesization.

Given an array of integers (length up to 5) and a target integer, determine whether any subset of the array can be combined using +, -, *, / and any parenthesization to produce exactly the target value. Division is exact rational division, not floating point. You must use at least one element and each element at most once.

“My first instinct was to just try all subsets and permutations and slap operators between them left to right, which completely ignores parenthesization.”

View Post

Jane Street·Data Scientist·Technical Phone Screen

Jun 2026

Phone screen for a Data Scientist role at Jane Street. One question, pure mental math and combinatorial reasoning, no calculator allowed. Felt deceptively simple at first glance.

You have 5 pumpkins. You weigh every possible pair on a scale, giving you 10 readings: 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30 pounds. You don't know which pair produced which reading. What is the total weight of all five pumpkins?

“My first instinct was to try solving for individual weights, which is a dead end without knowing which reading maps to which pair.”

View Post

Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview with a genuinely tricky algorithmic problem about merging text-editor diffs. The constraint that you can't materialize the document and must run in O(n+m) is what makes it hard. No fluff, just a meaty coding question.

Given two sequential insertion diffs (each a sorted list of position-value pairs), write a function that merges them into a single equivalent diff, without ever constructing or iterating over the underlying document. Must run in O(n+m) time where n and m are the sizes of the two diffs.

“This one took me a while to even understand what they were asking.”

View Post

Jane Street·Software Engineer·Onsite - Coding / Algorithms

Jun 2026

Jane Street SWE interview with a snake game BFS problem. Small grid, deceptively tricky state representation once you account for the snake's body changing as it moves.

Given a snake on an R x C grid with a known body configuration, find the minimum number of moves for the snake's head to reach an apple. The snake grows when it eats the apple, and a move into the current tail cell is legal on non-eating moves since the tail vacates simultaneously. Return -1 if unreachable.

“My first instinct was plain BFS on just the head position, which is completely wrong once the snake is longer than one cell.”

View Post