Why do I get StackOverFlowError when trying to DFS this graph?

Question

I am trying to write an algorithm that determines whether a graph is connected or not. I think my code is almost correct, although I keep getting StackOverFlowError. I personally think because there's a cycle in the graph I'm testing my algorithm with, my code doesn't understand that and comes in a loop. But I'm using an array to see if a node was already visited! So that should not happen! Anyways this is my code:

public int isConnected(String s) 
    {

        int in = nodes.indexOf(s);


        visited[in] = true;
        counter++;
        for(int i = 0; i < listOfChildren(s).size(); i++)
        {
            int ind = nodes.indexOf(listOfChildren(s).get(i));
            if(visited[ind] == false)
            {
                isConnected(listOfChildren(s).get(i));
            }

        }
        System.out.println(counter);
        if(counter == nodes.size())
            return 1;
        return 0;

    }

s is some node I begin with, nodes is an ArrayList of nodes and has the same size(5 in this case) as the array visited. In the beginning visited looks like this: [false false false false false], so none of the nodes was visited. listOfChildren() return an ArrayList of the children(not all of them, just the ones adjacent to the node) of a particular node. I am sure that listOfChildren() works, since I tested it 43545454 times.

Any help is appreciated(with minimum change of the code, if possible). Thanks.

UPDATE:

My graph is directed..

I define visited like this:

private boolean[] visited;

and I set everything in it to false in my constructor this code:

public void setUnvisited()
    {
        visited =  new boolean[nodes.size()];

        for(int i = 0; i < nodes.size(); i++)
        {
            visited[i] = false;
        }
    }

The nodes are strings! visited and nodes have the same length. That's why I can use nodes.indexOf(blabla) for the array visited.

UPDATE2:

This is how the graph looks like:

I'm sure the problem is after N3, the algorithm goes in a loop after N3, because it doesn't understand that N1 was already visited. I really don't understand why this happens!

UPDATE3

String have different names and there are no duplicates.. so for example indexOf(nodes.get(2)) gives 2 and not 0 or anything else..

The problem is that after visiting N3 the algorithm should stop and return 0 or 1, but I don't know how to do that :)

Answer 1

The reason you found this so difficult to track down is all the mutable state in your graph class. In particular you have count and visited , the latter being modified by both your methods isConnected and setUnvisited .

You depend on your array visited to tell you when to stop recursing, but accidentally reset the array on every recursive call through your call to listOfChildren .

One way around this to make it so that visited is not a class member. Here's a solution which modifies your code very little:

public boolean isConnected(String s) {
    int nVisited = isConnected(s, new boolean[nodes.size()], 0);
    return nVisited == nodes.size();
}

private int isConnected(String s, boolean[] visited, int counter) 
{

    int in = nodes.indexOf(s);


    visited[in] = true;
    counter++;
    for(int i = 0; i < listOfChildren(s).size(); i++)
    {
        int ind = nodes.indexOf(listOfChildren(s).get(i));
        if(visited[ind] == false)
        {
            counter = isConnected(listOfChildren(s).get(i), visited, counter);
        }

    }
    System.out.println(counter);
    return counter;
}

Since visited and counter are no longer shared, the bug you had is gone. This also solves another bug you had (but didn't notice yet) where only the first isConnected() call works--because in that case you weren't resetting visited or counter appropriately.

A cleaner implementation of the same idea as above:

public boolean isConnected(String s) {
    Set<String> visited = new HashSet<String>();
    isConnected(s, visited);
    return visited.size() == nodes.size();
}

private void isConnected(String s, Set<String> visited) 
{
    visited.add(s);
    for (String child : listOfChildren(s)) {
        if (!visited.contains(s)) {
            isConnected(child, visited);
        }
    }
}

I haven't actually tried to compile or run that, so there may be bugs, but you get the idea, I hope.

Answer 2

I made a small test program based on your updates, and it seems to work like a charm:

public class NodeTest
{
    static ArrayList<String> nodes = new ArrayList<String>();
    boolean visited[] = {false, false, false, false, false};

    int counter = 0;

    static HashMap<String, ArrayList<String>> childMap = new HashMap<String, ArrayList<String>>();

    static
    {
        nodes.add("N0");
        nodes.add("N1");
        nodes.add("N2");
        nodes.add("N3");
        nodes.add("N4");

        //N4 --> N2 --> N3 --> N1 <-- N0
        //       ^-------------+
        ArrayList<String> list = new ArrayList<String>();
        list.add("N2");
        childMap.put("N4", list); //N4 to N2

        list = new ArrayList<String>();
        list.add("N3"); 
        childMap.put("N2", list); //N2 to N3

        list = new ArrayList<String>();
        list.add("N1");
        childMap.put("N3", list); //N3 to N1

        list = new ArrayList<String>();
        list.add("N2");
        childMap.put("N1", list); //N1 to N2

        list = new ArrayList<String>();
        list.add("N1");
        childMap.put("N0", list); //N0 to N1
    }

    @Test
    public void test()
    {
        System.out.println("Is connected = " + isConnected("N0"));
    }

    public int isConnected(String s) 
    {
        System.out.println("Handling node " + s);

        int in = nodes.indexOf(s);


        visited[in] = true;
        counter++;
        for(int i = 0; i < listOfChildren(s).size(); i++)
        {
            int ind = nodes.indexOf(listOfChildren(s).get(i));
            if(visited[ind] == false)
            {
                System.out.println("Recursing into child " + listOfChildren(s).get(i));
                isConnected(listOfChildren(s).get(i));
            }
            else
            {
                System.out.println("Node " + listOfChildren(s).get(i) + " has already been visited");
            }

        }
        //System.out.println(counter);
        if(counter == nodes.size())
            return 1;
        return 0;

    }

    public ArrayList<String> listOfChildren(String s)
    {
        return childMap.get(s);
    }

}

The isConnected-method is same as yours, I just added some messages for logging. Output:

Handling node N0
Recursing into child N1
Handling node N1
Recursing into child N2
Handling node N2
Recursing into child N3
Handling node N3
Node N1 has already been visited
Is connected = 0

As expected, the graph is not connected (it's the same graph you drew on your question). If I change the starting node to N4 and change N1's only child to N0, the algorithm correctly recognizes the graph as connected. Based on this, I'd say the problem is either the listOfChildren returning something wacky or the indices used with visited (at some point, visited[in] = true marks some other index as visited than if(visited[ind] == false) is checking against when they should be the same?).

Answer 3

The real problem is, you are trying to represent a Node by a String. By doing that, you have to store the children of a Node somewhere else, listOfChildren . You also need to keep track of who you visited, boolean[] visited .

A node is now identified in two ways

1. listOfChildren uses the String representation "node1","node2",...
2. visited[] uses an index, the index in nodes .

Oho. Now, you must be sure, that every String representation has exactly one index in the nodes array. (There must be a one-to-one mapping of the two identifications.)

nodes.add("node");
nodes.add("node");
nodes.add("node");

return nodes.indexOf( nodes.get(2) );//what does this return? 0!

See the problem? There is one 'node' with three indices, or there are three nodes with the same name!

But I'm using an array to see if a node was already visited!

Maybe that is not the same 'node', do you mean the String 'node', or the index 'node'?

To fix this make one identification, one representation, for a node:

public class Node
{
  List<Node> children;
}

No String, or index needed! Just let node be a Node.