data_structures_free

net/datastructures/FindCycleDFS.java

@@ -0,0 +1,54 @@
+package net.datastructures;
+import java.util.Iterator;
+//begin#fragment FindCycleDFS
+/** This class specializes DFS to find a cycle. */
+//end#fragment FindCycleDFS
+ /* @author Roberto Tamassia, Michael Goodrich, Eric Zamore
+ */
+//begin#fragment FindCycleDFS
+public class FindCycleDFS<V, E> 
+  extends DFS<V, E, Object, Iterable<Position>> {
+  protected PositionList<Position> cycle; // sequence of edges of the cycle
+  protected boolean done;
+  protected Vertex<V> cycleStart;
+//end#fragment FindCycleDFS
+  /**
+   * Executes the DFS algorithm.
+   * @param info unused
+   * @return {@link Iterable} collection containing the vertices and
+   * edges of a cycle.
+   */
+//begin#fragment FindCycleDFS
+  public void setup() { 
+    cycle = new NodePositionList<Position>();
+    done = false;
+  }
+  protected void startVisit(Vertex<V> v) { cycle.addLast(v); }
+  protected void finishVisit(Vertex<V> v) {
+    cycle.remove(cycle.last());	// remove v from cycle
+    if (!cycle.isEmpty()) cycle.remove(cycle.last()); // remove edge into v from cycle
+  }
+  protected void traverseDiscovery(Edge<E> e, Vertex<V> from) { 
+    cycle.addLast(e); 
+  }
+  protected void traverseBack(Edge<E> e, Vertex<V> from) {
+    cycle.addLast(e);		// back edge e creates a cycle
+    cycleStart = graph.opposite(from, e);
+    cycle.addLast(cycleStart);	// first vertex completes the cycle
+    done = true;
+  }
+  protected boolean isDone() {  return done; } 
+  public Iterable<Position> finalResult(Iterable<Position> r) {
+    // remove the vertices and edges from start to cycleStart
+    if (!cycle.isEmpty()) {
+      for (Position<Position> p: cycle.positions()) {
+	if (p.element() == cycleStart)
+	  break;
+	cycle.remove(p);                     // remove vertex from cycle
+      }
+    }
+    return cycle; // list of the vertices and edges of the cycle 
+  }
+}
+//end#fragment FindCycleDFS
+