Write a program to determine if a string contains only unique characters.
Return true if it does and false otherwise.
The string may contain any of the 128 ASCII characters.
Characters are case-sensitive, e.g. 'a' and 'A' are considered different characters.
def has_unique_chars(str):
  return len(set(str))==len(str)
def has_unique_chars(str):
  return len(set(str))==len(str)