Print out all keys and values from a map in C++ （打印字典中所有的key和Value）
https://www.techiedelight.com/print-keys-values-map-cpp/
如果key不存在會如何？
https://stackoverflow.com/questions/10124679/what-happens-if-i-read-a-maps-value-where-the-key-does-not-exist
unordered_map erase in C++ STL
https://www.geeksforgeeks.org/unordered_map-erase-in-c-stl/