input: Write a function to compute the factorial of a number in Python.
  assert factorial_1(5) == 120
  assert factorial_1(0) == 1
  assert factorial_2(6) == 720
  assert factorial_2(3) == 6
  assert factorial_3(4) == 24
  assert factorial_3(7) == 5040
output: def factorial_1(n):
      if n == 0:
          return 1
      else:
          return n * factorial_1(n-1)
  
  def factorial_2(n):
      if n == 0:
          return 1
      else:
          return n * factorial_2(n-1)
  
  def factorial_3(n):
      if n == 0:
          return 1
      else:
          return n * factorial_3(n-1)