title: How do you a double factorial in python?
body: <p>I've been stucked on this question for a really long time.
  I've managed to do a single recursive factorial.</p>
  
  <pre><code>def factorial(n):
       if n == 0:
           return 1
       else:
           return n * factorial(n-1)
  </code></pre>
  
  <p>Double factorial
  For an even integer n, the double factorial is the product of all even positive integers less than or equal to n. For an odd integer p, the double factorial is the product of all odd positive integers less than or equal to p.</p>
  
  <p>If n is even, then <code>n!! = n*(n - 2)*(n - 4)*(n - 6)* ... *4*2</code></p>
  
  <p>If p is odd, then <code>p!! = p*(p - 2)*(p - 4)*(p - 6)* ... *3*1</code></p>
  
  <p>But I have no idea to do a double factorial. Any help?</p>