Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the logic errors pattern
The wrong operator, an inverted condition, a formula the right way round but backwards.
What went wrong: Counter.subtract() retains keys with counts of zero or negative numbers. The minus operator (-) removes non-positive counts.
The fix: Use the minus operator between Counters to prune zero and negative counts: dict(Counter(stock) - Counter(sold))
Remember: In Python Counter, c.subtract() keeps zero/negative counts, while c1 - c2 strips them.
The pattern behind it: logic errors.
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