Added iterative DFS algorithm.

+package org.onlab.graph;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.Stack;
+
+/**
+ * DFS graph search algorithm implemented via iteration rather than recursion.
+ */
+public class DepthFirstSearch<V extends Vertex, E extends Edge<V>>
+        extends AbstractGraphPathSearch<V, E> {
+
+    /**
+     * Graph edge types as classified by the DFS algorithm.
+     */
+    public static enum EdgeType {
+        TREE_EDGE, FORWARD_EDGE, BACK_EDGE, CROSS_EDGE
+    }
+
+    @Override
+    public SpanningTreeResult search(Graph<V, E> graph, V src, V dst,
+                                     EdgeWeight<V, E> weight) {
+        checkArguments(graph, src, dst);
+
+        // Prepare the search result.
+        SpanningTreeResult result = new SpanningTreeResult(src, dst);
+
+        // The source vertex has cost 0, of course.
+        result.updateVertex(src, null, 0.0, true);
+
+        // Track finished vertexes and keep a stack of vertexes that have been
+        // started; start this stack with the source on it.
+        Set<V> finished = new HashSet<>();
+        Stack<V> stack = new Stack<>();
+        stack.push(src);
+
+        while (!stack.isEmpty()) {
+            V vertex = stack.peek();
+            if (vertex.equals(dst)) {
+                // If we have reached our destination, bail.
+                break;
+            }
+
+            double cost = result.cost(vertex);
+            boolean tangent = false;
+
+            // Visit all egress edges of the current vertex.
+            for (E edge : graph.getEdgesFrom(vertex)) {
+                // If we have seen the edge already, skip it.
+                if (result.isEdgeMarked(edge)) {
+                    continue;
+                }
+
+                // Examine the destination of the current edge.
+                V nextVertex = edge.dst();
+                if (!result.hasCost(nextVertex)) {
+                    // If this vertex have not finished this vertex yet,
+                    // not started it, then start it as a tree-edge.
+                    result.markEdge(edge, EdgeType.TREE_EDGE);
+                    double newCost = cost + (weight == null ? 1.0 : weight.weight(edge));
+                    result.updateVertex(nextVertex, edge, newCost, true);
+                    stack.push(nextVertex);
+                    tangent = true;
+                    break;
+
+                } else if (!finished.contains(nextVertex)) {
+                    // We started the vertex, but did not yet finish it, so
+                    // it must be a back-edge.
+                    result.markEdge(edge, EdgeType.BACK_EDGE);
+                } else {
+                    // The target has been finished already, so what we have
+                    // here is either a forward-edge or a cross-edge.
+                    result.markEdge(edge, isForwardEdge(result, edge) ?
+                            EdgeType.FORWARD_EDGE : EdgeType.CROSS_EDGE);
+                }
+            }
+
+            // If we have not been sent on a tangent search and reached the
+            // end of the current scan normally, mark the node as finished
+            // and pop it off the vertex stack.
+            if (!tangent) {
+                finished.add(vertex);
+                stack.pop();
+            }
+        }
+
+        // Finally, but the paths on the search result and return.
+        result.buildPaths();
+        return result;
+    }
+
+    /**
+     * Determines whether the specified edge is a forward edge using the
+     * accumulated set of parent edges for each vertex.
+     *
+     * @param result search result
+     * @param edge   edge to be classified
+     * @return true if the edge is a forward edge
+     */
+    protected boolean isForwardEdge(DefaultResult result, E edge) {
+        // Follow the parent edges until we hit the edge source vertex
+        V target = edge.src();
+        V vertex = edge.dst();
+        Set<E> parentEdges;
+        while ((parentEdges = result.parents.get(vertex)) != null) {
+            for (E parentEdge : parentEdges) {
+                vertex = parentEdge.src();
+                if (vertex.equals(target)) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Graph search result which includes edge classification for building
+     * a spanning tree.
+     */
+    public class SpanningTreeResult extends DefaultResult {
+
+        protected final Map<E, EdgeType> edges = new HashMap<>();
+
+        /**
+         * Creates a new spanning tree result.
+         *
+         * @param src search source
+         * @param dst optional search destination
+         */
+        public SpanningTreeResult(V src, V dst) {
+            super(src, dst);
+        }
+
+        /**
+         * Returns the map of edge type.
+         *
+         * @return edge to edge type bindings
+         */
+        public Map<E, EdgeType> edges() {
+            return edges;
+        }
+
+        /**
+         * Indicates whether or not the edge has been marked with type.
+         *
+         * @param edge edge to test
+         * @return true if the edge has been marked already
+         */
+        boolean isEdgeMarked(E edge) {
+            return edges.containsKey(edge);
+        }
+
+        /**
+         * Marks the edge with the specified type.
+         *
+         * @param edge edge to mark
+         * @param type edge type
+         */
+        void markEdge(E edge, EdgeType type) {
+            edges.put(edge, type);
+        }
+
+    }
+
+}

+package org.onlab.graph;
+
+import org.junit.Test;
+
+import java.util.Set;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+import static org.onlab.graph.DepthFirstSearch.EdgeType;
+
+/**
+ * Test of the DFS algorithm.
+ */
+public class DepthFirstSearchTest extends AbstractGraphPathSearchTest {
+
+    @Override
+    protected DepthFirstSearch<TestVertex, TestEdge> graphSearch() {
+        return new DepthFirstSearch<>();
+    }
+
+    @Test
+    public void defaultGraphTest() {
+        executeDefaultTest(3, 6, 5.0, 12.0);
+        executeBroadSearch();
+    }
+
+    @Test
+    public void defaultHopCountWeight() {
+        weight = null;
+        executeDefaultTest(3, 6, 3.0, 6.0);
+        executeBroadSearch();
+    }
+
+    protected void executeDefaultTest(int minLength, int maxLength,
+                                      double minCost, double maxCost) {
+        g = new AdjacencyListsGraph<>(vertices(), edges());
+        DepthFirstSearch<TestVertex, TestEdge> search = graphSearch();
+
+        DepthFirstSearch<TestVertex, TestEdge>.SpanningTreeResult result =
+                search.search(g, A, H, weight);
+        Set<Path<TestVertex, TestEdge>> paths = result.paths();
+        assertEquals("incorrect path count", 1, paths.size());
+
+        Path path = paths.iterator().next();
+        System.out.println(path);
+        assertEquals("incorrect src", A, path.src());
+        assertEquals("incorrect dst", H, path.dst());
+
+        int l = path.edges().size();
+        assertTrue("incorrect path length " + l,
+                   minLength <= l && l <= maxLength);
+        assertTrue("incorrect path cost " + path.cost(),
+                   minCost <= path.cost() && path.cost() <= maxCost);
+
+        System.out.println(result.edges());
+        printPaths(paths);
+    }
+
+    public void executeBroadSearch() {
+        g = new AdjacencyListsGraph<>(vertices(), edges());
+        DepthFirstSearch<TestVertex, TestEdge> search = graphSearch();
+
+        // Perform narrow path search to a specific destination.
+        DepthFirstSearch<TestVertex, TestEdge>.SpanningTreeResult result =
+                search.search(g, A, null, weight);
+        assertEquals("incorrect paths count", 7, result.paths().size());
+
+        int[] types = new int[]{0, 0, 0, 0};
+        for (EdgeType t : result.edges().values()) {
+            types[t.ordinal()] += 1;
+        }
+        assertEquals("incorrect tree-edge count", 7,
+                     types[EdgeType.TREE_EDGE.ordinal()]);
+        assertEquals("incorrect back-edge count", 1,
+                     types[EdgeType.BACK_EDGE.ordinal()]);
+        assertEquals("incorrect cross-edge & forward-edge count", 4,
+                     types[EdgeType.FORWARD_EDGE.ordinal()] +
+                             types[EdgeType.CROSS_EDGE.ordinal()]);
+    }
+
+}