BFSPaths.java

import java.io.File;

public class BFSPaths
 {
    private static final int INFINITY = Integer.MAX_VALUE;
    private boolean[] marked;  // marked[v] = is a shortest path to this vertex known
    private int[] edgeTo;      // edgeTo[v] = previous edge on shortest s-v path
    private int[] distTo;      // distTo[v] = number of edges shortest s-v path
  
     // bfs from one source to other vertices
     public BFSPaths(Graph G, int s) {
        marked = new boolean[G.V()];
        distTo = new int[G.V()];
        edgeTo = new int[G.V()];
	bfs(G, s);
    }

    // Compute shortest path between source s and every other vertex in the graph 
     private void bfs(Graph G, int s) {
        Queue<Integer> q = new Queue<Integer>();
        for (int v = 0; v < G.V(); v++)
	    distTo[v] = INFINITY;
        distTo[s] = 0;
        marked[s] = true;
        q.enqueue(s);

        while (!q.isEmpty()) {
            int v = q.dequeue();
            for (int w : G.adj(v)) {
                if (!marked[w]) {
                    edgeTo[w] = v;
                    distTo[w] = distTo[v] + 1;
                    marked[w] = true;
                    q.enqueue(w);
                }
            }
        }
    }

     public boolean hasPathTo(int v) { return marked[v]; }
     
     public int distTo(int v) { return distTo[v]; }
     
     public Iterable<Integer> pathTo(int v) {
        if (!hasPathTo(v)) return null;
        Stack<Integer> path = new Stack<Integer>();
        int x;
        for (x = v; distTo[x] != 0; x = edgeTo[x])
            path.push(x);
        path.push(x);
        return path;
    }

    public static void main(String[] args) {
        File file = new File(args[0]);        
        Graph G = new Graph(file);
        int s = Integer.parseInt(args[1]);
        BFSPaths bfs = new BFSPaths(G, s);

        for (int v = 0; v < G.V(); v++) {
            if (bfs.hasPathTo(v)) {
                System.out.printf("%d to %d (%d):  ", s, v, bfs.distTo(v));
                for (int x : bfs.pathTo(v)) {
                    if (x == s) System.out.print(x);
                    else        System.out.print("-" + x);
                }
                System.out.println();
            }
            else {
                System.out.printf("%d to %d (-):  not connected\n", s, v);
            }
        }
    }
}