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

score 2 · Accepted Answer · answered Mar 26 '17 at 00:22

2

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));
    }
  }
}

answered Mar 26 '17 at 00:22

cookie_monster

121
5

Ahhhh of course. I had read that the only real difference between the two was the stack (DF) and the queue (BF), but was confused because my depth first solution had no reference to any stacks, and then I remembered that recursive calls put themselves on the program stack to keep track of each output :) I'll try this solution and get back to you – Richardweber32 Mar 26 '17 at 21:35
Works like a charm. Thank you – Richardweber32 Mar 26 '17 at 22:30

Breadth-First N-ary tree traversal

1 Answers1