Recursive double factorial P61384

Statement

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Write a recursive function that returns n!!.

Recall that n!! = n × (n − 2) × (n − 4) × …. For instance, 9!! = 9 × 7 × 5 × 3 × 1 = 945 and 8!! = 8 × 6 × 4 × 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≤ n ≤ 19.

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

Public test cases

Input/Output

double_factorial(9) → 945
double_factorial(8) → 384
double_factorial(1) → 1
double_factorial(0) → 1
double_factorial(19) → 654729075

Information