“…Recommender Systems, which aim to recommend potentially interested items for users and solve the information explosion problem, are playing critical roles in E-commerce sites (e.g., Amazon, JD.com, Alibaba) [10,20,41,42], videos sharing sites (e.g., YouTube) [7], picture sharing sites (e.g., Pinterest) [36], social networks (e.g., Facebook) [11,13] and so on. For example, in JD.com, one of the largest E-commerce sites in the world, the Recommender System serves more than 0.3 billion users in China, Thailand, Malaysia and other countries, and contributes billions of dollars for the Gross Merchandise Volume (GMV) (i.e., the total sales value for merchandise sold) each year.…”