untime complexity: `O(log(n))` -- approximate.

        >>> sd = SortedDict()
        >>> sd['c'] = 3
        >>> sd['a'] = 1
        >>> sd['b'] = 2
        >>> sd
        SortedDict({'a': 1, 'b': 2, 'c': 3})

        :param key: key for item
        :param value: value for item

        N)