Meta·Performance Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026Remote

Summary

Meta Production Engineer loop, and this one was almost entirely a single algorithmic problem about pairing dinosaur crossings across two sensor lines and computing speeds. PE-track specific apparently, so don't expect to see this if you're not in that loop. The follow-ups got progressively more annoying.

Questions Asked (1)

You have two sensor lines a fixed distance apart. Each line records timestamps when a dinosaur crosses it. Pair up crossings from line 1 and line 2 that represent the same dinosaur (within a reasonable time window), then compute the speed for each pair. Follow-ups include filtering by a speed threshold and handling multiple dinosaurs in flight simultaneously.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was brute force, match every line-1 timestamp against every line-2 timestamp and pick the closest.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints: sensor distance, expected speed range, timestamp precision, and whether multiple dinosaurs can cross simultaneously. Then propose a two-pointer merge algorithm that pairs crossings within a time window, computes speeds, and filters by threshold, while discussing trade-offs for handling multiple dinosaurs and performance optimizations.

Pro tip: Emphasize that the time window should be derived from the sensor distance and plausible dinosaur speeds, and mention that using a sliding window or two-pointer approach avoids O(n^2) complexity, which is crucial for high-throughput sensor data.

1. Clarify requirements and constraints

Ask about sensor distance, expected speed range, timestamp precision, and whether multiple dinosaurs can cross simultaneously. This defines the time window and pairing logic.

2. Design pairing algorithm

Use two pointers to merge the two sorted timestamp lists, pairing crossings within the computed time window. Handle edge cases like unmatched crossings or multiple dinosaurs by allowing one-to-many or many-to-many matches within the window.

3. Compute speeds and filter

For each valid pair, compute speed as distance divided by time difference. Apply a speed threshold filter to remove outliers or invalid pairs.

4. Address multiple dinosaurs and performance

Discuss strategies for simultaneous crossings, such as using a queue or interval tree, and analyze time/space complexity. Optimize for streaming data if needed.

Key Points to Mention

Two-pointer technique for O(n) pairing of sorted timestamps
Deriving the time window from sensor distance and plausible speed range
Handling multiple dinosaurs with one-to-many matching or interval overlap
Speed calculation and threshold filtering for outlier removal
Time and space complexity analysis, including streaming considerations
Edge cases: unmatched crossings, clock skew, and sensor noise

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