1 Introduction

2 Problems and Approaches

2.1 Opinion-based PQA

2.2 Extraction-based PQA

2.3 Retrieval-based PQA

2.4 Generation-based PQA

3 Challenges and Solutions

3.1 Subjectivity

3.2 Answer Reliability & Answer Ability

3.3 Multi-type Resources

3.4 Low-resource

4 Prospects and Future Directions

5 Conclusions