input: Write a function to calculate the factorial of a given number using recursion in Python. The function should take an integer `n` as input and return the factorial of `n`.
  
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(10)==3628800
  assert factorial(3)==6
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)