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