Go through the tree - but step by step

I have the following code to move a tree (pre-order):

public void traverse(Node node) {
    visit(node);
    for (Node child : node.getChildren()) {
        traverse(child);
    } 
}

I would like a step-by-step move. something like Iterator, so that the client (the caller), which may be another application, can control the progress. (for example: in the user interface, we have a "Next" button, and by clicking on this button, we should visit the next node)

my current solution looks something like this:

List<Node> nodes = new ArrayList<Node>();
collectNodes(root, nodes);
Iterator<Node> it = nodes.iterator();
// do my job.
...

public void collectNodes(Node node, List<Node> nodes) {
    nodes.add(node);
    for (Node child : node.getChildren()) {
        collectNodes(child, nodes);
    } 
}

As you can see in the code, I visit all the nodes (in collectNodes) to collect and place them in a list in pre-order format.

I was wondering if there is any solution without this extra (collectNodes) iteration?

Regards, Mohammad