CC4.2.java

    public boolean isReachable(DirectedGraphNode p, DirectedGraphNode q) {
        if (p == null || q == null)
            return false;
        if (p == q)
            return true;
        HashSet<DirectedGraphNode> visited = new HashSet<DirectedGraphNode>();
        Queue<DirectedGraphNode> queue = new LinkedList<DirectedGraphNode>();
        queue.add(p);
        visited.add(p);
        while (!queue.isEmpty()) {
            DirectedGraphNode node = queue.poll();
            for (int i = 0; i < node.neighbors.size(); i++) {
                DirectedGraphNode nbr = node.neighbors.get(i);
                if (nbr == q)
                    return true;
                if (!visited.contains(nbr)) {
                    queue.add(nbr);
                    visited.add(nbr);
                }
            }
        }
        return false;
    }