Clarify the event log format and trip definition, then propose a hash map keyed by license plate to track each plate's state (entry, road, exit). Iterate through events, updating state and incrementing a trip counter when a plate completes the entry→road→exit sequence, handling out-of-order or incomplete events gracefully.
Pro tip: Mention that you would validate the sequence and handle edge cases like duplicate entries or exits without prior entry, showing attention to data integrity. Also, discuss time/space complexity (O(n) time, O(p) space for p plates) to demonstrate efficiency awareness.
Ask about the log format (e.g., timestamp, plate, event type), whether events are ordered, and if a plate can have multiple trips. Confirm that a complete trip requires exactly entry, then road, then exit in order.
Use a hash map to store the current state of each plate (e.g., 0=no entry, 1=entered, 2=on road, 3=exited). Iterate through events, updating state and counting trips when a plate transitions from road to exit.
Consider invalid sequences (e.g., exit without entry), duplicate events, and plates that never complete a trip. Decide whether to ignore or log such anomalies.
State that the solution runs in O(n) time and O(p) space, where n is number of events and p is number of unique plates. Discuss potential optimizations if needed.
Walk through a small example to verify correctness, including a complete trip and an incomplete one. Mention testing edge cases like empty log or single event.
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