The serialization of a binary tree follows a level order traversal, where ‘#‘ signifies a path terminator where no node exists below.

Here‘s an example:

   1
  /  2   3
    /
   4
         5

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public TreeNode copyNode(TreeNode node)
    {
        if(node==null)
        {
            return null;
        }
        TreeNode nodeCopy=new TreeNode(node.val);
        List<TreeNode> listOriginal=new LinkedList<TreeNode>();
        List<TreeNode> listCopy=new LinkedList<TreeNode>();
        listOriginal.add(node);
        listCopy.add(nodeCopy);
        while(listOriginal.size()>0)
        {
            TreeNode original=listOriginal.get(0);
            TreeNode copy=listCopy.get(0);
            if(original.left!=null)
            {
                copy.left=new TreeNode(original.left.val);
                listOriginal.add(original.left);
                listCopy.add(copy.left);
            }
            if(original.right!=null)
            {
                copy.right=new TreeNode(original.right.val);
                listOriginal.add(original.right);
                listCopy.add(copy.right);
            }
            listOriginal.remove(0);
            listCopy.remove(0);
        }
        return nodeCopy;
    }
    public List<TreeNode> generateTrees(int start,int end)
    {
        List<TreeNode> result=new LinkedList<TreeNode>();
        TreeNode root=null;
        if(start>end)
        {
            result.add(null);
            return result;
        }
        for(int i=start;i<=end;i++)
        {
            
            List<TreeNode> listLeft=generateTrees(start,i-1);
            List<TreeNode> listRight=generateTrees(i+1,end);
            for(TreeNode left:listLeft)
            {
                for(TreeNode right:listRight)
                {
                    root=new TreeNode(i);
                    root.left=copyNode(left);
                    root.right=copyNode(right);
                    result.add(root);
                }
            }
        }
        return result;
    }
    public List<TreeNode> generateTrees(int n) {
        if(n<0)
        {
            return new LinkedList<TreeNode>();
        }
        return generateTrees(1,n);
    }
}