https://www.geeksforgeeks.org/linked-list-set-1-introduction/
Binary search tree:
https://www.geeksforgeeks.org/binary-search-tree-set-1-search-and-insertion/
Python Dictionary get():
http://www.runoob.com/python/att-dictionary-get.html
Python Dictionary:
https://www.tutorialspoint.com/python/python_dictionary.htm
Python Dictionary items():
http://www.runoob.com/python/att-dictionary-items.html