def string_unique(stri):
    """
    Given a string, determine if it has all unique characters.
    """
    if len(stri) > 128:
        return False

    char_set = [False] * 128
    for char in stri:
        val = ord(char) #ord return the unicode value of the character
        if char_set[val]:
            return False
        char_set[val] = True
    return True