input: Write a function to take a positive integer as input and use a while loop to output the numbers counting down to 1. After reaching 1, print "Done!".
  
  assert countdown(6)=="6\n5\n4\n3\n2\n1\nDone!"
  assert countdown(3)=="3\n2\n1\nDone!"
  assert countdown(1)=="1\nDone!"
  assert countdown(0)=="Done!"
  assert countdown(10)=="10\n9\n8\n7\n6\n5\n4\n3\n2\n1\nDone!"
output: def countdown(n):
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result