Find Successor & Predecessor in BST

First, we use recursive way.

Successor

 public class Solution {
     public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
         if (root == null) {
             return null;
         }
         if (root.val <= p.val) {
             return inorderSuccessor(root.right, p);
         } else {
             TreeNode left = inorderSuccessor(root.left, p);
             return left == null ? root : left;
         }
     }
 }

Predecessor

 public class Solution {
     public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
         if (root == null) {
             return null;
         }
         if (root.val >= p.val) {
             return inorderSuccessor(root.left, p);
         } else {
             TreeNode right = inorderSuccessor(root.right, p);
             return right == null ? root : right;
         }
     }
 }