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: factorial calls itself with n - 1 forever (5, 4, 3 … 0, -1, -2 …). There is no point where it stops, so Python gives up.
The fix: add if n <= 1: return 1 at the top.
Remember: every function that calls itself needs a stopping case, called the base case.
The pattern behind it: mutation and copying bugs.
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