input: Write a function to convert a list of strings into a single concatenated string.
  assert list_to_string(['Hello', 'World', '!']) == 'HelloWorld!'
  assert list_to_string(['Python', 'is', 'awesome']) == 'Pythonisawesome'
  assert list_to_string(['a', 'b', 'c', 'd']) == 'abcd'
output: def list_to_string(lst):
      return ''.join(lst)