graph.rb

require 'rgl/implicit'
require 'rgl/traversal'

vertices = ["one", "two", "three"]

g = RGL::ImplicitGraph.new do |g|
  g.vertex_iterator { |b| vertices.map{|v| b.call(v) } }
  g.adjacent_iterator { |x, b| b.call( vertices[(vertices.index(x) - 1).abs] ) }
  g.directed = true
end

t = g.bfs_search_tree_from("one")

puts t.has_vertex?("two")   # true
puts t.has_vertex?("three") # false

http://blog.willj.net/2015/09/25/how-to-find-if-two-nodes-are-connected-in-an-rgl-graph/