input: Write a function to create a Python program that takes in a positive integer and uses a while loop to output the numbers counting down to 1. Return the string "Done!" after the loop completes.
  
  assert count_down(6) == "6\n5\n4\n3\n2\n1\nDone!"
  assert count_down(3) == "3\n2\n1\nDone!"
  assert count_down(1) == "1\nDone!"
  assert count_down(0) == "Done!"
output: def count_down(n):
      result = ""
      while n > 0:
          result += str(n) + "\n"
          n -= 1
      result += "Done!"
      return result