Jane Street·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Jane Street technical screen for a Data Scientist role. One coding problem, pretty involved, took up most of the time. The problem sounds straightforward until you actually think about the constraints.

Questions Asked (1)

You have two event streams, each producing (company, trade_id) pairs. Two streams are considered equivalent if, for every company, the ordered sequence of that company's trade IDs is the same in both streams (interleaving across companies can differ). Write a function that checks equivalence while treating the inputs as streams: single pass, early termination the moment a contradiction is found, and memory bounded by the number of unmatched in-flight events rather than total feed length.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was to group by company, collect all trade IDs per company, then compare.

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AI HintsAI Generated

Suggested Approach

Clarify the streaming constraints and equivalence definition, then propose a solution using per-company queues to track unmatched trade IDs, ensuring single-pass processing and early termination. Discuss memory bounds and trade-offs, and consider edge cases like duplicate trade IDs and company reappearance.

Pro tip: Emphasize that memory is bounded by the maximum number of in-flight events per company, not total stream length, and that early termination occurs as soon as a mismatch is detected. Mention that using a hash map of queues allows O(1) amortized operations per event.

1. Clarify requirements and constraints

Confirm that streams are infinite, single-pass, and that equivalence requires identical per-company ordered sequences. Discuss memory bounds and early termination expectations.

2. Design data structures

Use a hash map from company to a queue of trade IDs from stream A that are waiting to be matched. For stream B, check against the queue for that company.

3. Define processing logic

For each event from A, enqueue the trade ID for its company. For each event from B, if the queue for that company is non-empty, dequeue and compare; if mismatch, terminate. If empty, buffer the B event in a separate queue for that company.

4. Handle interleaving and termination

Process events from both streams in any order, but ensure that when one stream ends, the other is drained and all queues are empty for equivalence. Terminate early on any mismatch.

5. Analyze complexity and edge cases

Discuss time O(n) and memory O(k) where k is max in-flight events. Address duplicate trade IDs, companies with no trades, and stream exhaustion.

Key Points to Mention

Single-pass processing with O(1) amortized time per event using hash map and queues.
Memory bounded by the number of unmatched in-flight events, not total stream length.
Early termination as soon as a contradiction is found (mismatch or stream ends with non-empty queues).
Handling of duplicate trade IDs correctly by preserving order.
Trade-offs: buffering one stream's events when the other hasn't produced matching trades.
Edge cases: empty streams, companies appearing in only one stream, and stream exhaustion.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.