input: Write a python function to sort the characters in a string in alphabetical order.
  assert sort_string('hello') == 'ehllo'
  assert sort_string('python') == 'hnopty'
  assert sort_string('world') == 'dlorw'
  assert sort_string('aabbcc') == 'aabbcc'
  assert sort_string('') == ''
output: def sort_string(s):
      return ''.join(sorted(s))