Determine if a string has all unique characters.
Using an array we can hash characters to their ASCII value. For each character, we determine the ascii value, look up the array slot and check if the character is flagged. If it is not flagged, it is the first time the character has been seen and we mark it and continue to the next character. If we reach a slot that is flagged, the string does not have all unique characters. If we reach the end of the string, we have determined the string has all unique characters.