Jane Street Software Engineer Interview Questions

Jane Street·Software Engineer·Technical Phone Screen

Jul 2026

Jane Street software engineer interview with a probability brain teaser. The kind of question where you either see the structure quickly or spend five minutes going in circles.

In a three-player dice game where each person rolls a standard 6-sided die and the highest roll wins (with ties broken by re-rolling among tied players), your two friends have both rolled a 4. You roll next. What is the probability that you eventually win?

“I stared at this longer than I'd like to admit.”

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Jane Street·Software Engineer·Recruiter / HR Screen

Jul 2026

Recruiter screen for a software engineer role at Jane Street. Just one question from what was shared, a standard motivation question, but Jane Street puts real weight on these so it's worth thinking through.

Why Jane Street? Walk us through what draws you to this firm and this role specifically, with concrete examples from your background.

“They want specifics, not vibes.”

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

Jul 2026

Jane Street software engineering interview where I built a two-player piece-dropping game engine from scratch and then got asked how I'd productionize it as a library or API. The design follow-up was the real interview.

You've just coded a working game engine under time pressure as a single script. If you had to turn this into a library other engineers depend on, or expose it as a network API, what would you change and what would you add?
How would you add a new rule variant, like diagonal wins or a third player, without breaking existing users of the library?
If this became a network API, would you make the service stateful or stateless? What are the trade-offs?
Once other teams are depending on the library, how do you roll out a breaking change to the public interface?
If you only had time to write five tests before shipping, which behaviors would you pin down first?

“This is where I fumbled a bit.”

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Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview with a two-part coding problem centered on building a collapsible code editor feature. The problem was more involved than it looked at first glance, especially the nesting independence requirement in part two.

Given a list of lines representing source code where each line has at most one brace as its last character, write a function that returns a mapping from each opening brace's line index to its matching closing brace's line index, in a single pass.
Extend the brace-matching solution with a toggle and getText interface that supports collapsing and expanding individual brace pairs, where each pair's collapsed state is fully independent regardless of nesting.

“Stack-based solution, pretty standard once you see it.”

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Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview, coding round focused on a trading systems problem. Pretty domain-flavored but the core was just a sorting exercise with a twist at the end. Left feeling okay about it but the follow-up question made me second-guess myself.

Given an array of trade execution records that may arrive out of order, implement a function that sorts them into a canonical ordering: first by timestamp ascending, then by symbol lexicographically, then by trade ID lexicographically as a final tiebreaker.
How would you modify your approach if trade IDs are no longer guaranteed to be unique, meaning duplicate or replayed records could appear in the feed?

“Felt straightforward at first and it basically was.”

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Jane Street·Software Engineer·Technical Phone Screen

Jun 2026

Jane Street SWE interview with a pretty gnarly streaming systems question. The core problem was interesting but the follow-up variant is what really tested whether you'd actually thought through the edge cases.

You have a stream transformer that reconstructs a sparse code-time matrix. Input is batches of (timestamp, code, value) tuples and output is one (timestamp, row) pair per timestamp, with missing entries filled as -1. Now suppose the tuples within a batch are no longer sorted by code, only timestamps are guaranteed non-decreasing across batches. How do you modify the transformer to still emit correct rows, and what are the time and space tradeoffs?

“This is the kind of question where the base case feels manageable and then the variant exposes whether you actually understand the data flow.”

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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.”

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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.”

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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.”

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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.”

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