迭代器yield问题，有高手可以帮解决下吗？
下面是一个实现二叉树的类，其中继承IEnumenable<T>接口用GetEnumerator()实现foreach迭代功能，可是得到的结果却不对，为什么呢？

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;



namespace BinaryTree

{

    public class Tree<TItem>:IEnumerable<TItem> where TItem : IComparable<TItem>

    {

        public Tree(TItem nodeValue)

        {

            this.NodeData = nodeValue;

            this.LeftTree = null;

            this.RightTree = null;

        }



        public void Insert(TItem newItem)

        {

            TItem currentNodeValue = this.NodeData;

            if (currentNodeValue.CompareTo(newItem) > 0)

            {

                if (this.LeftTree == null)

                {

                    this.LeftTree = new Tree<TItem>(newItem);

                }

                else

                {

                    this.LeftTree.Insert(newItem);

                }

            }

            else

            {

                if (this.RightTree == null)

                {

                    this.RightTree = new Tree<TItem>(newItem);

                }

                else

                {

                    this.RightTree.Insert(newItem);

                }

      &nb