edge.rb

  def longest_road(visited_roads = [], skip_roads = [])
    visited_roads << self

    # left roads
    unvisited_left_roads = left_roads - visited_roads - skip_roads
    left_road_lenghts = unvisited_left_roads.map do |road|
      lenghts, new_visited_roads = road.longest_road(visited_roads, unvisited_left_roads)
      visited_roads = (visited_roads + new_visited_roads).uniq
      lenghts
    end

    # right roads
    unvisited_right_roads = right_roads - visited_roads - skip_roads
    right_road_lenghts = unvisited_right_roads.map do |road|
      lenghts, new_visited_roads = road.longest_road(visited_roads, unvisited_right_roads)
      visited_roads = (visited_roads + new_visited_roads).uniq
      lenghts
    end

    longest_road = 1 + (left_road_lenghts.max or 0) + (right_road_lenghts.max or 0)
    return [longest_road, visited_roads]
  end