Representing Graphs with Adjacency Lists in Python Dictionaries

# Representing an undirected graph using an adjacency list (dictionary of lists)
graph = {
    'A': ['B', 'C'],
    'B': ['A', 'D', 'E'],
    'C': ['A', 'F'],
    'D': ['B'],
    'E': ['B', 'F'],
    'F': ['C', 'E']
}

def add_edge(g, u, v):
    """Adds an undirected edge between nodes u and v."""
    if u not in g:
        g[u] = []
    if v not in g:
        g[v] = []
    g[u].append(v)
    g[v].append(u)

def print_graph(g):
    for node, neighbors in g.items():
        print(f"{node}: {', '.join(neighbors)}")

print("Initial Graph:")
print_graph(graph)

# Adding a new node and edge
add_edge(graph, 'G', 'A')
print("
Graph after adding edge 'G-A':")
print_graph(graph)

# Checking for connectivity
print(f"
Neighbors of B: {graph['B']}")
print(f"Is A connected to D? {'D' in graph['A']}")
print(f"Is A connected to B? {'B' in graph['A']}")

# A simple depth-first search (DFS) traversal example
def dfs(g, start_node, visited=None):
    if visited is None:
        visited = set()
    visited.add(start_node)
    print(start_node, end=" ")
    for neighbor in g.get(start_node, []):
        if neighbor not in visited:
            dfs(g, neighbor, visited)

print("
DFS traversal from 'A':")
dfs(graph, 'A')
print()