Given a string, write a function to determine if it contains only unique characters. If all characters are unique, return True; otherwise, return False.

Example:

Input: "abcde"
Output: True

The function should return True because all characters are unique.

Input: "aabcde"
Output: False

The function should return False because 'a' is repeated.

Constraints:

• The function should be case sensitive, meaning that 'A' and 'a' are considered different characters.
• The function should handle any printable ASCII character.
• The solution should not use additional data structures like arrays for character counts if possible.