reorg and add code for second edition

10_greedy_algorithms/ruby/01_set_covering.rb

@@ -0,0 +1,31 @@
+require "set"
+
+# You pass an array in, and it gets converted to a Set.new.
+states_needed = Set.new(%w(mt wa or id nv ut ca az))
+
+stations = {}
+stations["kone"] = Set.new(%w(id nv ut))
+stations["ktwo"] = Set.new(%w(wa id mt))
+stations["kthree"] = Set.new(%w(or nv ca))
+stations["kfour"] = Set.new(%w(nv ut))
+stations["kfive"] = Set.new(%w(ca az))
+
+final_stations = Set.new
+
+until states_needed.empty?
+  best_station = nil
+  states_covered = Set.new
+
+  stations.each do |station, states|
+    covered = states_needed & states
+    if covered.length > states_covered.length
+      best_station = station
+      states_covered = covered
+    end
+  end
+
+  states_needed -= states_covered
+  final_stations << best_station
+end
+
+p final_stations