Lỗi duyệt cây binary tree trong java

[TrauGia1702 (Trâu Gia Gia)]

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package Tree;

/**
 *
 * @author think
 */
public class BinaryTree
{
    public static void main(String[] args) {
        BTree bt = new BTree();
        
        bt.add(6);
        bt.add(4);
        bt.add(7);
        bt.add(9);
       
        bt.print();
    }
}

class Node
    {
        int value;
        Node left;
        Node right;
        
        Node(int value)
        {
            this.value = value;
            
            left = null;
            right = null;
        }
    }


class BTree 
{
    
    private Node root;
    
    public BTree()
    {
        this.root = null;
    }
    
    public Node getRoot()
    {
        return root;
    }
    public void add(int value)
    {
        addRecursion(this.root, value);
    }
    
    private void addRecursion(Node current, int value)
    {
        if(current == null)
        {
            current =  new Node(value);
            System.out.println("add: " + current.value);
        }
        else
        {
            if(value < current.value)
            {
                addRecursion(current.left, value);
            }
            else if(value > current.value)
            {
                addRecursion(current.right, value);
            }
        }
        return;
    }
    
    public void print()
    {
        PreoderTravesal(this.root);
    }
    
    // preoder recursiion
    private void PreoderTravesal(Node root)
    {
        while(root != null)
        {
            System.out.println("node: " + root.value);
            PreoderTravesal(root.left);
            PreoderTravesal(root.right);
        }
    }
    
    
   
}

sao mình không duyt được dcaya nhỉ, lỗi tàn hình.