**** INTRODUCTION: 

Implement some basic data structures:

* Implemented: 
 -linked list
 -hash table: now it is built on top of linked list

* To-be implemented: 
 -stack/queue
 -BST/RBT
 -depth first search tree: traversal

Code structure: 
 * Each structure is implemented in the corresponding header file. 
 * Main file is used for testing of implemented structures.


**** TO-DO: 

- Linked list: generalize linked list to any data type
- Hash table should contain <key, value> pair
- HT: first node is always dummy, use it to hold some useful info?

- implement in C++ class
- add error handling

- Other data structures