From the world’s rapid urbanization, people tend to migrate from rural areas (focusing on agricultural economy) to the urban areas (emphasizing on industrial economy) over the last few decades (e.g. see Figures 1 and 2). According to statistical data, the population of large cities around the world has increased considerably. In 2018, the UN stated that 55% of the world's population lives in various metropolitan areas, and this figure will increase to 68% within the next 30 years [1]. Typically, the highly populated urban area often encounters various economic and social problems.
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