def unique(string):
    """
    Algorithm to determine if a string's characters are all unique
    :param string: input string (ASCII character set)
    :return: Boolean
    """
    # Special Case, cannot be greater than 128 characters
    if len(string) > 128:
        return False

    # Set up "Empty" list for observed characters
    char_set = [False for _ in range(128)]

    for char in string:
        val = ord(char)  # ord converts a character to its unicode value
        if char_set[val]:  # char has already been observed
            return False
        char_set[val] = True

    return True