Leetcode: Inorder Successor in BST

Given a binary search tree and a node in it, find the in-order successor of that node in the BST.

Note: If the given node has no in-order successor in the tree, return null.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
12         ArrayList<TreeNode> prev = new ArrayList<TreeNode>();
13         ArrayList<TreeNode> res = new ArrayList<TreeNode>();
14         prev.add(null);
15         res.add(null);
16         inorder(root, p, prev, res);
17         return res.get(0);
18     }
19     
20     public void inorder(TreeNode root, TreeNode p, ArrayList<TreeNode> prev, ArrayList<TreeNode> res) {
21         if (root == null) return;
22         inorder(root.left, p, prev, res);
23         if (prev.get(0) == p) {
24             res.set(0, root);
25         }
26         prev.set(0, root);
27         inorder(root.right, p, prev, res);
28     }
29 }