Recursive double factorial

Write a recursive function that returns @n@ $!!$ .

Recall that $n!! = n \times (n - 2) \times (n - 4) \times \dots$ . For instance, $9!! = 9 \times 7 \times 5 \times 3 \times 1 = 945$ and $8!! = 8 \times 6 \times 4 \times 2 = 384$ . By definition, $0!! = 1!! = 1$ .

Interface

C++	`int double_factorial(int n);`
C	`int double_factorial(int n);`
Java	`public static int doubleFactorial(int n);`
Python	`double_factorial(n) # returns int`
	`double_factorial(n: int) -> int`

Precondition

Assume $0\le$ @n@ $\le 19$ .

Observation

You only need to submit the required procedure; your main program will be ignored.

Problem information

Author: Unknown
Translator: Salvador Roura

Generation: 2026-01-25T11:09:02.414Z