Add class 7 topics, resources, and challenges: maps and hash tables

Author: neptunius

Class7.md

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+## Class 7: Maps & Hash Tables
+
+### Topics
+- Abstract data types: [map (dictionary, associative array)][map]
+- Concrete data structures: [hash table]
+- [Hash functions], [collision resolution], [load factor], [dynamic resizing]
+
+### Resources
+- Review Make School's [hash table slides]
+- Watch Make School's [hash table video lecture]
+- Play with VisuAlgo's [interactive hash table visualization][visualgo hash table]
+
+### Challenges
+- Add new features to improve `HashTable` class using [hash table starter code]:
+    - Add `size` property that tracks the number of hash table entries in constant time
+    - Implement `load_factor` - return the [load factor], the ratio of number of entries to buckets
+    - Implement `_resize` - perform [dynamic resizing] when `load_factor` exceeds `0.75` after an insertion (`set` is called with a new `key`) and rehash all key-value entries
+    - Run `python hashtable.py` to test `HashTable` class instance methods on a small example
+    - Run `pytest hashtable_test.py` to run the [hash table unit tests] and fix any failures
+- Annotate methods with complexity analysis of running time and space (memory)
+
+### Stretch Challenges
+- Implement an alternative hash table [collision resolution] strategy instead of [separate chaining] (popular variants include [linear probing], [quadratic probing], and [double hashing])
+- Write additional test cases to expand the [hash table unit tests] to ensure your collision resolution strategy is robust
+
+
+[map]: https://en.wikipedia.org/wiki/Associative_array
+[hash table]: https://en.wikipedia.org/wiki/Hash_table
+[hash functions]: https://en.wikipedia.org/wiki/Hash_function
+[load factor]: https://en.wikipedia.org/wiki/Hash_table#Key_statistics
+[dynamic resizing]: https://en.wikipedia.org/wiki/Hash_table#Dynamic_resizing
+[collision resolution]: https://en.wikipedia.org/wiki/Hash_table#Collision_resolution
+[separate chaining]: https://en.wikipedia.org/wiki/Hash_table#Separate_chaining
+[linear probing]: https://en.wikipedia.org/wiki/Linear_probing
+[quadratic probing]: https://en.wikipedia.org/wiki/Quadratic_probing
+[double hashing]: https://en.wikipedia.org/wiki/Double_hashing
+
+[hash table slides]: slides/HashTables.pdf
+[hash table video lecture]: https://www.youtube.com/watch?v=nLWXJ6IDKmQ
+[visualgo hash table]: https://visualgo.net/hashtable
+
+[hash table starter code]: source/hashtable.py
+[hash table unit tests]: source/hashtable_test.py

ReadMe.md

@@ -15,7 +15,7 @@
 |   4   |    Monday, October 30  | [Arrays & Linked Lists](Class4.md)         |
 |   5   | Wednesday, November 1  | [Lists, Stacks & Queues](Class5.md)        |
 |   6   |    Friday, November 3  | [Call Routing Project](Class6.md)          |
-|   7   |    Monday, November 6  | Map & Hash Table              |
+|   7   |    Monday, November 6  | [Maps & Hash Tables](Class7.md)            |
 |   8   | Wednesday, November 8  | Set & Circular Buffer         |
 |   9   |    Friday, November 10 | Trees                         |
 |  10   |    Monday, November 13 | Tree Traversals               |