def is_unique(sentence):
    """ 1.1 Is Unique: Implement an algorithm to determine if a string has
    all unique characters. What if you cannot use additional data structures?
    Complexity: O(n) time, O(n) space
    """
    h = set([])
    for c in sentence:
        if c in h:
            return False
        h.add(c)
    return True