input: Write a function to understand the concept of recursion in Python by defining a recursive function that calculates the factorial of a given number. The function should return the factorial of the input number. If the input number is 1, the function should return 1 directly.
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(1)==1
  assert factorial(4)==24
  assert factorial(2)==2
  assert factorial(6)==720
output: def factorial(n):
      if n == 1:
          return 1
      else:
          return n * factorial(n - 1)