reorg and add code for second edition

09_dijkstras_algorithm/ruby/01_dijkstras_algorithm.rb

@@ -0,0 +1,69 @@
+# the graph
+graph = {}
+graph["start"] = {}
+graph["start"]["a"] = 6
+graph["start"]["b"] = 2
+
+graph["a"] = {}
+graph["a"]["fin"] = 1
+
+graph["b"] = {}
+graph["b"]["a"] = 3
+graph["b"]["fin"] = 5
+
+graph["fin"] = {}
+
+# the costs table
+costs = {}
+costs["a"] = 6
+costs["b"] = 2
+costs["fin"] = Float::INFINITY
+
+# the parents table
+parents = {}
+parents["a"] = "start"
+parents["b"] = "start"
+parents["fin"] = nil
+
+@processed = []
+
+def find_lowest_cost_node(costs)
+  lowest_cost = Float::INFINITY
+  lowest_cost_node = nil
+  # Go through each node.
+  costs.each do |node, cost|
+    # If it's the lowest cost so far and hasn't been processed yet...
+    if cost < lowest_cost && !@processed.member?(node)
+      # ... set it as the new lowest-cost node.
+      lowest_cost = cost
+      lowest_cost_node = node
+    end
+  end
+  lowest_cost_node
+end
+
+# Find the lowest-cost node that you haven't processed yet.
+node = find_lowest_cost_node(costs)
+# If you've processed all the nodes, this while loop is done.
+until node.nil?
+  cost = costs[node]
+  # Go through all the neighbors of this node.
+  neighbors = graph[node]
+  neighbors.keys.each do |n|
+    new_cost = cost + neighbors[n]
+    # If it's cheaper to get to this neighbor by going through this node...
+    if costs[n] > new_cost
+      # ... update the cost for this node.
+      costs[n] = new_cost
+      # This node becomes the new parent for this neighbor.
+      parents[n] = node
+    end
+  end
+  # Mark the node as processed.
+  @processed << node
+  # Find the next node to process, and loop.
+  node = find_lowest_cost_node(costs)
+end
+
+puts "Cost from the start to each node:"
+puts costs