public static boolean isAcyclic(Digraph digraph) {
int order = digraph.order();
if (order == 0) return true;
Set spanned = new HashSet(order);
DepthFirstStampSearch dfs = new DepthFirstStampSearch(digraph, digraph.vertexIterator().next());
for (Iterator i = digraph.vertexIterator(); i.hasNext(); ) {
Object dfsRoot = i.next();
if (spanned.contains(dfsRoot)) continue;
dfs.reset(dfsRoot);
Map dfsOrders = dfs.traverse(new HashMap(digraph.order()));
for (Iterator j = dfsOrders.entrySet().iterator(); j.hasNext(); ) {
Map.Entry entry = (Map.Entry) j.next();
Object origin = entry.getKey();
DepthFirstStampSearch.OrderPair orgOrders =
(DepthFirstStampSearch.OrderPair) entry.getValue();
spanned.add(origin);
for (ArcIterator k = digraph.outgoingIterator(origin); k.hasNext(); ) {
k.next();
Object dst = k.getDestination();
DepthFirstStampSearch.OrderPair dstOrders =
(DepthFirstStampSearch.OrderPair) dfsOrders.get(dst);
if (dstOrders.getPostOrder() > orgOrders.getPostOrder()) return false;
}
}
if (dfsOrders.size() == order) break;
}
return true;
}
public static boolean isTree(Digraph digraph) {
Object root = null;
for (Iterator i = digraph.vertexIterator(); i.hasNext(); ) {
Object vertex = i.next();
int inSize = digraph.incomingSize(vertex);
if (inSize == 0) {
root = vertex;
break;
}
}
// not a tree - no vertex with 0 in-degree
if (root == null) return false;
// try to reach all vertices from the root candidate
BreadthFirstSearch traversal = new BreadthFirstSearch(digraph, root);
while (traversal.isValidTree() && traversal.hasNext()) traversal.next();
// not a tree - one of vertices has been seen more than once by the BFS
if (!traversal.isValidTree()) return false;
// has every vertex been reached?
Set seenVertices = traversal.getSeenVertices();
for (Iterator i = digraph.vertexIterator(); i.hasNext(); )
if (!seenVertices.contains(i.next())) return false;
// all tests are passed - good!
return true;
}