def inverse_transform(self, y, lengths=None):
        """Return label strings.

        Args:
            y: label id matrix.
            lengths: sentences length.

        Returns:
            list: list of list of strings.
        """
        y = np.argmax(y, -1)
        inverse_y = [self._label_vocab.id2doc(ids) for ids in y]
        if lengths is not None:
            inverse_y = [iy[:l] for iy, l in zip(inverse_y, lengths)]

        return inverse_y