Representing Graphs with Adjacency Lists

from collections import defaultdict

# --- Undirected Graph Representation ---
def create_undirected_graph(edges):
    graph = defaultdict(list)
    for u, v in edges:
        graph[u].append(v)
        graph[v].append(u) # Add edge in both directions for undirected
    return graph

# Example: Social network (users are nodes, friendships are edges)
social_network_edges = [('Alice', 'Bob'), ('Bob', 'Charlie'), ('Alice', 'David'), ('David', 'Eve')]
social_graph = create_undirected_graph(social_network_edges)

print("
--- Undirected Social Network ---")
for node, neighbors in social_graph.items():
    print(f"{node}: {neighbors}")
# Expected Output:
# Alice: ['Bob', 'David']
# Bob: ['Alice', 'Charlie']
# Charlie: ['Bob']
# David: ['Alice', 'Eve']
# Eve: ['David']

# --- Directed Graph Representation ---
def create_directed_graph(edges):
    graph = defaultdict(list)
    for u, v in edges:
        graph[u].append(v) # Only add edge from u to v
    return graph

# Example: Website navigation (pages are nodes, links are edges)
website_links = [('Home', 'About'), ('Home', 'Products'), ('About', 'Contact'), ('Products', 'Item1'), ('Item1', 'Home')]
website_graph = create_directed_graph(website_links)

print("
--- Directed Website Navigation ---")
for node, neighbors in website_graph.items():
    print(f"{node}: {neighbors}")
# Expected Output:
# Home: ['About', 'Products']
# About: ['Contact']
# Products: ['Item1']
# Item1: ['Home']
# Contact: [] (Contact might not appear if it has no outgoing links and isn't an 'origin' node in any edge)

# Function to find neighbors of a node
def get_neighbors(graph, node):
    return graph.get(node, [])

print(f"
Neighbors of 'Alice' in social network: {get_neighbors(social_graph, 'Alice')}")
print(f"Neighbors of 'Home' in website navigation: {get_neighbors(website_graph, 'Home')}")