Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the mutation and copying bugs pattern
Assignment does not copy, a shallow copy still shares its contents, and mutating methods return None.
What went wrong: a set has no order, so turning it back into a list can give any order.
The fix: return list(dict.fromkeys(track_ids))
Remember: use a set to check membership. When order matters, use dict.fromkeys().
The pattern behind it: mutation and copying bugs.
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<iframe src="https://trybughunt.com/embed/a1a2c277-77ef-4319-a390-ec2a7cffaaf2" title="Debugging challenge: Removing duplicate tracks changes the order" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe>
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