78 - Hashtable can not avoid Hash Collision --> Linked List can solve. Hashtable is data type that contain (key, value) pair. Hashtable can be call as various name in different languages. Eg. Dictionaries, Objects - Sets, Tuples... 116 - JS is single thread --> call stack WHY SHOULD USE A DATA STRUCTURE OR ALGORITHM eventhough it has its drawbacks? 200 - BFS use Queue data structure (FIFO) to traverser through the tree - facebook/linked in closest connection, 204 - DFS with Pre/In/Post Order - to see if something exists.