data structure hat store data as key value pairs
to build hashtable class and define add, get and contains methods for it
hash: time O(n), space O(1) add: time O(1), space O(1) get: time O(1) space O(1) contains: time O(1) space O(1)
hash: method returns Index in the collection for that key
add: method that hash the key, and add the key and value pair to the table, handling collisions as needed.
get: method returns Value associated with that key in the table.
contains: method returns Boolean, indicating if the key exists in the table already.