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Adjacency List

In the first assignment, we created a graph using an adjacency matrix:

	0	1	2	3	4
0	False	True	False	False	True
1	True	False	True	True	True
2	False	True	False	True	False
3	False	True	True	False	True
4	True	True	False	True	False

Through the rest of this course, we'll primarily be using an adjacency list instead. An adjacency list stores a list of vertices for each vertex that indicates where the connections are:


0	connects with	1	4
1	connects with	0	2	3	4
2	connects with	1	3
3	connects with	1	2	4
4	connects with	0	1	3

Assignment

Let's rebuild our Graph class using an adjacency list.

Complete the constructor:
- It should create an empty dictionary called graph as a data member.
Complete the add_edge method. It takes two vertices as inputs, and adds an edge to the adjacency list (the dictionary):
- Be sure to map both vertices to each other, it's a bidirectional edge.
- Handle the case where a set for a vertex doesn't exist yet.
- The resulting graph maps vertices to a set of all other vertices they share an edge with. For example:

result = {0: {1, 4}, 1: {0, 2, 3, 4}, 2: {1, 3}, 3: {1, 2, 4}, 4: {0, 1, 3}}