input: Write a python function to check if all characters in a string are unique.
  assert check_unique("abcdefg")==True
  assert check_unique("hello")==False
  assert check_unique("")==True
  assert check_unique("12345")==True
  assert check_unique("112233")==False
output: def check_unique(s):
      return len(s) == len(set(s))