input: Write a function to count down from a given positive integer to 1 using a while loop. The function should take in a positive integer and print the numbers counting down to 1. If the given integer is 0 or negative, the function should print "Invalid input".
  assert countdown(6)==None
  assert countdown(0)=="Invalid input"
  assert countdown(-5)=="Invalid input"
  assert countdown(3)==None
output: def countdown(n):
      if n <= 0:
          print("Invalid input")
      else:
          while n > 0:
              print(n)
              n -= 1