traverse all leaf nodes in a tree C#

Question

I am trying to traverse all leaf nodes in a tree using queue. But I cannot get any output.

class MyNode<T>
{
    public T Data { get; set; }
    public MyNode<T> Parent { get; set; }
    public List<MyNode<T>> Children = new List<MyNode<T>>();
    public MyNode(T data, MyNode<T> parent)
    {
        Data = data;
        Parent = parent;
    }

    public override string ToString()
    {
        if (Children == null) return Data.ToString();
        return string.Format("{0} {1} ", Data.ToString(), Children.ToString());
    }

}

A node can have any number of children. And here is what I wrote to print all leaf nodes out. I cannot get anything, I think only the last line Console.WriteLine(""); was executed, but I cannot figure out why.

    public static void PrintSentence(MyNode<string> root)
    {
        if (root == null)   // Return when the tree is empty.
            return;

        Queue<MyNode<string>> nodeQueue = new Queue<MyNode<string>>();
        nodeQueue.Enqueue(root);

        MyNode<string> currentNode = root;

        while (nodeQueue.Count != 0)
        {
            currentNode = nodeQueue.Peek();
            nodeQueue.Dequeue();

            if (currentNode.Children == null)   // Print strings only when the current node is a leaf node.
                Console.Write(currentNode.Data + " ");

            for (int i = 0; i < currentNode.Children.Count(); i++)
                nodeQueue.Enqueue(currentNode.Children[i]);
        }

        Console.WriteLine("");

    }

Thanks for any help. The tree class is this, actually I cannot find my debug window anywhere... I only wrote the PrintSentence method, and other things was written by someone else.

class Tree<T>
{
    public MyNode<T> Root { get; set; }
    public Tree(MyNode<T> root) { Root = root; }
    public override string ToString()
    {
        if (Root == null) return "";
        return Root.ToString();
    }
}

Answer 1

Generic solution:

public static class Hierarchy
{
    /// <summary>
    /// Gets the collection of leafs (items that have no children) from a hierarchical collection
    /// </summary>
    /// <typeparam name="T">The collection type</typeparam>
    /// <param name="source">The sourceitem of the collection</param>
    /// <param name="getChildren">A method that returns the children of an item</param>
    /// <returns>The collection of leafs</returns>
    public static IEnumerable<T> GetLeafs<T>(T source, Func<T, IEnumerable<T>> getChildren)
    {
        if (!getChildren(source).Any())
        {
            yield return source;
        }
        else
        {
            foreach (var child in getChildren(source))
            {
                foreach (var subchild in GetLeafs(child, getChildren))
                {
                    yield return subchild;
                }
            }
        }
    }
}

Usage:

    var leafs = Hierarchy.GetLeafs(root, (element) => element.Children);

Answer 2

Separate node traversal and traversal action like this:

I have choose recursion, because the deep of recusrion for trees is not usually problem and you don't need to much memory for queueu.

public static class MyNodeExt<T>
{
   IEnumerable<T> TraverseLeafs<T>(this MyNode<T> node)
   {
       if (node.Children.Count == 0)
           yield return node;
       else
       {
           foreach(var child in root.Children)
           {
               foreach(var subchild in child.TraverseLeafs())
               {
                   yield return subchild;
               }
           } 
       }
   }
}

And separate traversal action:

public static void PrintSentence(MyNode<string> root)
{
    foreach(var node in root.TraverseLeafs())
    {
        Console.Write(node .Data + " ");
    }       
}

Answer 3

You'll need to replace this line

if (currentNode.Children == null)

with this

if (currentNode.Children.Count == 0)

This will check if the list has no elements (no children). Since you always initialize your list, it will won't be null to begin with, even when it is empty.