/**
* Determines whether the edge-weighted digraph {@code G} has a topological
* order and, if so, finds such an order.
* @param G the edge-weighted digraph
*/
public Topological(EdgeWeightedDigraph G) {
EdgeWeightedDirectedCycle finder = new EdgeWeightedDirectedCycle(G);
if (!finder.hasCycle()) {
DepthFirstOrder dfs = new DepthFirstOrder(G);
order = dfs.reversePost();
}
}
// by finding a cycle in predecessor graph
private void findNegativeCycle() {
int V = edgeTo.length;
EdgeWeightedDigraph spt = new EdgeWeightedDigraph(V);
for (int v = 0; v < V; v++)
if (edgeTo[v] != null)
spt.addEdge(edgeTo[v]);
EdgeWeightedDirectedCycle finder = new EdgeWeightedDirectedCycle(spt);
cycle = finder.cycle();
}
/**
* Returns a negative cycle, or {@code null} if there is no such cycle.
* @return a negative cycle as an iterable of edges,
* or {@code null} if there is no such cycle
*/
public Iterable<DirectedEdge> negativeCycle() {
for (int v = 0; v < distTo.length; v++) {
// negative cycle in v's predecessor graph
if (distTo[v][v] < 0.0) {
int V = edgeTo.length;
EdgeWeightedDigraph spt = new EdgeWeightedDigraph(V);
for (int w = 0; w < V; w++)
if (edgeTo[v][w] != null)
spt.addEdge(edgeTo[v][w]);
EdgeWeightedDirectedCycle finder = new EdgeWeightedDirectedCycle(spt);
assert finder.hasCycle();
return finder.cycle();
}
}
return null;
}
/**
* Unit tests the {@code EdgeWeightedDirectedCycle} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
// create random DAG with V vertices and E edges; then add F random edges
int V = Integer.parseInt(args[0]);
int E = Integer.parseInt(args[1]);
int F = Integer.parseInt(args[2]);
EdgeWeightedDigraph G = new EdgeWeightedDigraph(V);
int[] vertices = new int[V];
for (int i = 0; i < V; i++)
vertices[i] = i;
StdRandom.shuffle(vertices);
for (int i = 0; i < E; i++) {
int v, w;
do {
v = StdRandom.uniform(V);
w = StdRandom.uniform(V);
} while (v >= w);
double weight = StdRandom.uniform();
G.addEdge(new DirectedEdge(v, w, weight));
}
// add F extra edges
for (int i = 0; i < F; i++) {
int v = StdRandom.uniform(V);
int w = StdRandom.uniform(V);
double weight = StdRandom.uniform(0.0, 1.0);
G.addEdge(new DirectedEdge(v, w, weight));
}
StdOut.println(G);
// find a directed cycle
EdgeWeightedDirectedCycle finder = new EdgeWeightedDirectedCycle(G);
if (finder.hasCycle()) {
StdOut.print("Cycle: ");
for (DirectedEdge e : finder.cycle()) {
StdOut.print(e + " ");
}
StdOut.println();
}
// or give topologial sort
else {
StdOut.println("No directed cycle");
}
}
