Inorder Successor in BST

public class Solution {
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
        return helper(root, p, null);
    }
    public TreeNode helper(TreeNode root, TreeNode p, TreeNode pre) {
        if (root == p) {
            if (root.right != null) {
                TreeNode tmp = root.right;
                while (tmp.left != null) {
                    tmp = tmp.left;
                }
                return tmp;
            } else {
                return pre;
            }
        }
        if (root.val < p.val && root.right != null) {
            return helper(root.right, p, pre);
        } else if (root.val > p.val && root.left != null) {
            return helper(root.left, p, root);
        }
        return null;
    }
}