面试必刷TOP101：33、二叉树的镜像

   public TreeNode Mirror (TreeNo服务器托管网de pRoot) {
        if(pRoot == null){
            return null;
        }
        TreeNode root = new TreeNode(pRoot.val);
        root.left = Mirror(pRoot.right);
        root.right = Mirror(pRoot.left);
        return root;
    }