input: Write a function to calculate the factorial of a given number. If the input number is non-negative, return the factorial of that number. If the input is negative, return None.
  def factorial(n):
   if n < 0:
    return None
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact
  
  assert factorial(5) == 120
  assert factorial(0) == 1
  assert factorial(-3) == None
  assert factorial(10) == 3628800
  assert factorial(1) == 1
output: def factorial(n):
   if n < 0:
    return None
   fact = 1
   for i in range(2,n+1):
    fact = fact * i
   return fact