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Headway·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Coding round at Headway for a software engineer role. The problem looked like a straightforward parsing task but the speeding logic had a second condition that tripped me up a bit.

Questions Asked (1)

Q1

You're given camera log entries from a segmented road system. Each entry records a vehicle plate and a timestamp when it crosses a segment boundary. Implement a function that returns all plates of vehicles caught speeding, where speeding means either a single segment averaged 130 km/h or more, or at least two separate segments each averaged 120 km/h or more.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The parsing part was fine, I sorted by timestamp per plate and computed speed as 36000 divided by the time delta.

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AI HintsAI Generated

Suggested Approach

First, clarify the input format and constraints, then design a solution that groups log entries by vehicle plate and sorts timestamps to compute segment speeds. Use a hash map to track per-vehicle segment speeds and apply the speeding criteria, ensuring O(n log n) time due to sorting.

Pro tip: Mention edge cases like multiple vehicles with the same plate, missing timestamps, or non-uniform segment lengths, and discuss how to handle them gracefully. Also, consider if the log entries are already sorted by time to optimize.

1. Clarify requirements and assumptions

Ask about the log entry format, segment lengths, time units, and whether entries are sorted. Confirm the exact speeding criteria and output format.

2. Design data structures

Use a hash map to group entries by plate, storing a list of (timestamp, segment_id) pairs. Sort each list by timestamp to compute speeds between consecutive entries.

3. Compute segment speeds and apply criteria

For each vehicle, iterate through sorted entries, calculate speed for each segment, and track if any segment >=130 km/h or count segments >=120 km/h. Mark vehicle as speeding if criteria met.

4. Handle edge cases and optimize

Consider missing data, duplicate timestamps, and non-uniform segment lengths. If input is large, discuss memory and time trade-offs, and potential streaming approach.

5. Test and validate

Walk through a small example to verify logic, and mention unit tests for boundary conditions like exactly 130 km/h or exactly two segments at 120 km/h.

Key Points to Mention

  • Hash map for grouping by plate and sorting timestamps
  • Time complexity: O(n log n) due to sorting, space O(n)
  • Speeding criteria: single segment >=130 km/h OR at least two segments >=120 km/h
  • Edge cases: missing segments, duplicate plates, non-uniform segment lengths
  • Potential optimization if logs are pre-sorted by time
  • Clear variable naming and modular code for readability

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