HashTable

Use HC as index to quickly query some key in the hash table.

Pro:
Quick lookup, insert and remove O(1) (on average)
Challenge:
1. Collisions: values with same hashcode
    1) Solution 1: linear probing: insert. Just put it in the next open spot.
        Has to search for subsequent space for values. May be a problem when the hash table gets full.
        Random probing is an alternative.
    2) Solution 2: separate chaining. Just keep a list at each spot.
        This solution is usually preferred. But drawbacks.
2. Resizing: resize if gets too full (70%)
    requires create a new table, new hash function, and reinsert everything.
3. Ordering data.
    Hash Table don't have order within structure.

Hash Set vs. Hash Map
1. Hash Set: java.util
    class hashset<E>
    boolean add(E e)
    boolean contains(Object o)
    Just tells you if an item is in the set. Perfect for a dictionary.

2. Hash Map: java.util
    class hashmap<K,V>
    V get(Object key)
    V put(K key, V value)
    Stores both a key and some data associated with the key.