Double Factorial

Create a function that takes a number num and returns its double factorial. Mathematically, the formulas for double factorial are as follows.

If num is even:

num !! = num (num - 2)(num - 4)(num - 6) ... (4)(2)

If num is odd:

num !! = num (num - 2)(num - 4)(num - 6) ... (3)(1)

If num = 0 or num = -1, then num !! = 1 by convention.

Examples

double_factorial(0) ➞ 1

double_factorial(2) ➞ 2

double_factorial(9) ➞ 945
# 9*7*5*3*1 = 945

double_factorial(14) ➞ 645120

Notes

• Assume all input values are greater than or equal to -1.
• Try to solve it with recursion.
• Double factorial is not the same as factorial * 2.