Breadth-First N-ary tree traversal

Question

I'm coding up an N-ary tree representation of file system hierarchies, which contains some information about the directories/files. Each node in the tree consists of a parent node, and a list of its children (if any) and is contained within a seperate Tree object. This isn't the most eloquent method of implementing a tree as far as i'm aware, but I'm far enough into the project where it isn't worth going back.

public class TreeNode {

    private FileSystemEntry data;
    private TreeNode parent;
    private ArrayList<TreeNode> children;
    private boolean directory; //separates files from folders (files have no children) 

The tree structure is defined as its own separate object, since there will be several trees.

public class DirectoryTree {

    private TreeNode Root;
    private int numNodes;
    private TreeNode Focus;

I understand that I will need to use a queue to add each node to while i traverse its children (or something similar).

Here's a Depth first recursive solution that prints the names of each file/directory, just for reference.

public void PrintTreeNames() {

    PrintTreeNames(this.Root);

}

private void PrintTreeNames(TreeNode n) {

    if (!n.isDirectory()) {
        System.out.println(n.getData().getName());
    } else {
        for (int i = 0; i < n.getChildren().size(); i++) {
            PrintTreeNames(n.getChildren().get(i));
        }
        System.out.println(n.getData().getName());
    }

}

I feel like it should only be a small modification to get from depth first to breadth first but I can't seem to get my head round it

Answer 1

Create the queue initially with just the root node, process the queue until it is empty. To process a node output it first, then add all of it's children to the queue:

public void PrintTreeNames() {

  Queue<TreeNode> queue = new LinkedList<TreeNode>();
  queue.add(this.root);
  TreeNode current;
  while ((current = queue.poll()) != null) {
    PrintTreeNames(current, queue);
  }
}

private void PrintTreeNames(TreeNode n, Queue<TreeNode> queue) {

  System.out.println(n.getData().getName());
  if (n.isDirectory()) {
    for (int i = 0; i < n.getChildren().size(); i++) {
      queue.add(n.getChildren().get(i));
    }
  }
}