Second largest node in the BST

 1 public class Solution {
 2     public Node secondLargest(Node root) {
 3         return helper(root, null);
 4     }
 5     
 6     private Node helper(Node root, Node previous) {
 7         if (root.right != null) {
 8             return helper(root.right, root);
 9         }
10         
11         if (root.left != null) {
12             return rightMost(root.left);
13         }
14         
15         return previous;
16     }
17     
18     private Node rightMost(Node node) {
19         while (node.right != null) {
20             node = node.right;
21         }
22         return node;
23     }
24 」