Model Graphs with Adjacency Lists in Python

class Graph:
    def __init__(self, directed=False):
        self.graph = {} # Dictionary: {node: set_of_neighbors}
        self.directed = directed

    def add_node(self, node):
        if node not in self.graph:
            self.graph[node] = set()

    def add_edge(self, node1, node2):
        self.add_node(node1) # Ensure nodes exist
        self.add_node(node2)
        self.graph[node1].add(node2)
        if not self.directed:
            self.graph[node2].add(node1) # For undirected graphs

    def get_neighbors(self, node):
        return self.graph.get(node, set())

    def get_all_nodes(self):
        return list(self.graph.keys())

    def __str__(self):
        return "\
".join([f"{node}: {sorted(list(neighbors))}" for node, neighbors in self.graph.items()])

# Example Usage: Undirected Graph (Social Network)
social_network = Graph(directed=False)
social_network.add_edge("Alice", "Bob")
social_network.add_edge("Bob", "Charlie")
social_network.add_edge("Alice", "David")
social_network.add_edge("Eve", "Bob")

print("--- Social Network (Undirected) ---")
print(social_network)
print(f"Alice's neighbors: {social_network.get_neighbors('Alice')}")
print(f"Bob's neighbors: {social_network.get_neighbors('Bob')}\
")

# Example Usage: Directed Graph (Web Links)
web_links = Graph(directed=True)
web_links.add_edge("Page A", "Page B")
web_links.add_edge("Page A", "Page C")
web_links.add_edge("Page B", "Page D")
web_links.add_edge("Page C", "Page B")

print("--- Web Links (Directed) ---")
print(web_links)
print(f"Pages linked from Page A: {web_links.get_neighbors('Page A')}")
print(f"Pages linked from Page B: {web_links.get_neighbors('Page B')}")