input: Write a python function to check if a string has all unique characters.
  assert is_unique_string("abcdef") == True
  assert is_unique_string("aabbcc") == False
  assert is_unique_string("123456") == True
  assert is_unique_string("112233") == False
output: def is_unique_string(s):
      return len(s) == len(set(s))