def add_undirected_edge(vertex_a, vertex_b, weight)
      add_edge(vertex_a, vertex_b, weight)
      add_edge(vertex_b, vertex_a, weight)
    end
def add_undirected_edge(vertex_a, vertex_b, weight)
      add_edge(vertex_a, vertex_b, weight)
      add_edge(vertex_b, vertex_a, weight)
    end
Add undirected edge (vertex_a, vertex_b) to the graph with given weight