def get_adjacent_vertices(source)
      adjacent_array = []
      if @edges.key?(source)
        adjacent_array = @edges[source].map { |dst, _weight| dst }
      end
      Set.new(adjacent_array)
    end
def get_adjacent_vertices(source)
      adjacent_array = []
      if @edges.key?(source)
        adjacent_array = @edges[source].map { |dst, _weight| dst }
      end
      Set.new(adjacent_array)
    end
Returns the set of vertices v_i where edge (source, v_i) is in the graph