LinkedIn·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

LinkedIn SWE interview with a vehicle trip tracking problem. The question was cleaner than it looked at first but the edge cases are where they really want to see how you think.

Questions Asked (1)

Given a log of vehicle events (each with a license plate and an event type of ENTRY, ROAD, or EXIT), implement a function that counts the total number of completed trips across all vehicles, where a valid trip is exactly ENTRY followed by ROAD followed by EXIT in order.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just count sequences naively and I almost missed that the same plate can have multiple trips.

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AI HintsAI Generated

Suggested Approach

Use a hash map to track the current state of each vehicle based on its license plate, updating the state as events are processed. When a vehicle completes the sequence ENTRY -> ROAD -> EXIT, increment the trip count and reset its state. This ensures O(n) time and O(k) space, where k is the number of unique vehicles.

Pro tip: Clarify edge cases upfront, such as multiple ENTRY events without EXIT or events out of order, and explain how your solution handles them. This shows attention to detail and robustness.

1. Understand the problem and constraints

Restate the problem to ensure clarity: count trips where events for a vehicle occur in exact order ENTRY, ROAD, EXIT. Ask about input format, event ordering, and whether events are guaranteed to be in chronological order.

2. Choose data structures

Select a hash map to store the current state of each vehicle (e.g., mapping license plate to an integer representing the last valid event in the sequence). This allows O(1) updates per event.

3. Define state transitions

Define valid transitions: from no state or after EXIT, an ENTRY sets state to 1; from state 1, a ROAD sets state to 2; from state 2, an EXIT increments trip count and resets state to 0. Invalid events reset the state to 0.

4. Iterate through events and update

Process each event in order, updating the state map according to the transitions. Count completed trips when an EXIT is valid.

5. Analyze complexity and edge cases

Discuss time complexity O(n) and space O(k). Mention edge cases: multiple ENTRYs, missing ROAD, EXIT without ENTRY, and how your solution handles them.

Key Points to Mention

Use of hash map for O(1) state lookup per vehicle
State machine approach to track valid sequences
Handling of invalid or out-of-order events by resetting state
Time complexity O(n) and space complexity O(k) where k is number of unique vehicles
Edge cases: multiple ENTRYs, missing ROAD, EXIT without ENTRY
Assumption that events are processed in chronological order

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