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;
        }
    }
}