public class KosarajuSharirSCC {
private boolean[] marked; // marked[v] = has vertex v been visited?
private int[] id; // id[v] = id of strong component containing v
private int count; // number of strongly-connected components
/**
* Computes the strong components of the digraph <tt>G</tt>.
* @param G the digraph
*/
public KosarajuSharirSCC(Digraph G) {
// compute reverse postorder of reverse graph
DepthFirstOrder dfs = new DepthFirstOrder(G.reverse());
// run DFS on G, using reverse postorder to guide calculation
marked = new boolean[G.V()];
id = new int[G.V()];
for (int v : dfs.reversePost()) {
if (!marked[v]) {
dfs(G, v);
count++;
}
}
// check that id[] gives strong components
assert check(G);
}
// DFS on graph G
private void dfs(Digraph G, int v) {
marked[v] = true;
id[v] = count;
for (int w : G.adj(v)) {
if (!marked[w]) dfs(G, w);
}
}
/**
* Returns the number of strong components.
* @return the number of strong components
*/
public int count() {
return count;
}
/**
* Are vertices <tt>v</tt> and <tt>w</tt> in the same strong component?
* @param v one vertex
* @param w the other vertex
* @return <tt>true</tt> if vertices <tt>v</tt> and <tt>w</tt> are in the same
* strong component, and <tt>false</tt> otherwise
*/
public boolean stronglyConnected(int v, int w) {
return id[v] == id[w];
}
/**
* Returns the component id of the strong component containing vertex <tt>v</tt>.
* @param v the vertex
* @return the component id of the strong component containing vertex <tt>v</tt>
*/
public int id(int v) {
return id[v];
}
// does the id[] array contain the strongly connected components?
private boolean check(Digraph G) {
TransitiveClosure tc = new TransitiveClosure(G);
for (int v = 0; v < G.V(); v++) {
for (int w = 0; w < G.V(); w++) {
if (stronglyConnected(v, w) != (tc.reachable(v, w) && tc.reachable(w, v)))
return false;
}
}
return true;
}
/**
* Unit tests the <tt>KosarajuSharirSCC</tt> data type.
*/
public static void main(String[] args) {
In in = new In(args[0]);
Digraph G = new Digraph(in);
KosarajuSharirSCC scc = new KosarajuSharirSCC(G);
// number of connected components
int M = scc.count();
StdOut.println(M + " components");
// compute list of vertices in each strong component
Queue<Integer>[] components = (Queue<Integer>[]) new Queue[M];
for (int i = 0; i < M; i++) {
components[i] = new Queue<Integer>();
}
for (int v = 0; v < G.V(); v++) {
components[scc.id(v)].enqueue(v);
}
// print results
for (int i = 0; i < M; i++) {
for (int v : components[i]) {
StdOut.print(v + " ");
}
StdOut.println();
}
}
}