  The chain-structured long short-term memory (LSTM) has showed to be effective
in a wide range of problems such as speech recognition and machine translation.
In this paper, we propose to extend it to tree structures, in which a memory
cell can reflect the history memories of multiple child cells or multiple
descendant cells in a recursive process. We call the model S-LSTM, which
provides a principled way of considering long-distance interaction over
hierarchies, e.g., language or image parse structures. We leverage the models
for semantic composition to understand the meaning of text, a fundamental
problem in natural language understanding, and show that it outperforms a
state-of-the-art recursive model by replacing its composition layers with the
S-LSTM memory blocks. We also show that utilizing the given structures is
helpful in achieving a performance better than that without considering the
structures.
