input: Write a function to calculate the factorial of a given number. If the given number is 0, then return 1.
  def factorial(n):
   if n == 0:
    return 1
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact
  
  assert factorial(7) == 5040
  assert factorial(0) == 1
  assert factorial(5) == 120
  assert factorial(3) == 6
  assert factorial(10) == 3628800
output: def factorial(n):
   if n == 0:
    return 1
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact