graph.py

class Graph:
    def __init__(self):
        self.vertices = {}

    def add_vertex(self, vertex):
        if vertex not in self.vertices:
            self.vertices[vertex] = []

    def add_edge(self, vertex1, vertex2):
        if vertex1 in self.vertices and vertex2 in self.vertices:
            self.vertices[vertex1].append(vertex2)
            self.vertices[vertex2].append(vertex1)

    def display(self):
        for vertex in self.vertices:
            print(f"{vertex}: {', '.join(map(str, self.vertices[vertex]))}")

# Example Usage:
graph = Graph()
graph.add_vertex(1)
graph.add_vertex(2)
graph.add_vertex(3)
graph.add_edge(1, 2)
graph.add_edge(2, 3)

graph.display()