二叉树的镜像

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

 1 /*二叉树的镜像*/
 2     public TreeNode mirrorTree(TreeNode root) {
 3         if (root == null) {
 4             return null;
 5         }
 6         TreeNode leftNode = mirrorTree(root.right);
 7         TreeNode rightNode = mirrorTree(root.left);
 8         root.left = leftNode;
 9         root.right = rightNode;
10         return root;
11     }