def get_edge_weight(source, destination)
      if contains_edge?(source, destination)
        @edges[source][destination]
      else
        Float::INFINITY
      end
    end
def get_edge_weight(source, destination)
      if contains_edge?(source, destination)
        @edges[source][destination]
      else
        Float::INFINITY
      end
    end
Returns the weight of directed edge (source, destination),
 or returns Float::INFINITY if no such edge exists