input: Write a function to calculate the factorial of a given non-negative integer. If the input number is 0, return 1.
  
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  
  assert factorial(0)==1
  assert factorial(5)==120
  assert factorial(7)==5040
  assert factorial(1)==1
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)