Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the off-by-one errors pattern
The loop that runs one time too many, the slice that drops the last item, the index that starts in the wrong place.
What went wrong: the idea is right, but range(n) adds up 0 to n - 1 instead of 1 to n. The total is n too small.
The fix: expected = sum(range(1, n + 1))
Remember: an impossible answer, like -1 here, often means a range is off by one.
The pattern behind it: off-by-one errors.
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