R:如何初始化空的二叉树？

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    
    public TreeNode(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}

public class BinaryTree {
    TreeNode root;
    
    public BinaryTree() {
        this.root = null;
    }
    
    public static void main(String[] args) {
        BinaryTree tree = new BinaryTree();
        // 初始化空的二叉树
        tree.root = null;
    }
}