It will be useful to have a helper function to get quick access to the nodes (aka vertices) in the graph that are adjacent to a given node.
Complete the adjacent_nodes(self, node) method. It takes a node (an integer) as input and returns a set of all the adjacent nodes. For example:
graph = Graph()
graph.add_edge(0, 1)
graph.add_edge(0, 2)
graph.add_edge(1, 3)
graph.add_edge(2, 3)
adjacent_nodes = graph.adjacent_nodes(1)
# {0, 3}