二叉搜索树中FindParent，无需在接口上添加方法

I am given an interface to implement methods for binary search tree. The interface is below. I can not add methods to this interface for my implementation. I got other methods working but for the parent method the implementation I have requires to add method on the interface however I can not do so. How to implement parent method without requiring to add another method on the interface. Thank you for your help.

    public interface BinarySearchTree<K extends Comparable<K>, V> {
        public NonemptyBinarySearchTree<K, V> insert(K key, V value);
        public int size();
        public V searchForKey(K key);
        public K max() throws EmptyTreeException;
        public K min() throws EmptyTreeException;
        public K parent(K key);
        public NonemptyBinarySearchTree<K, V> deleteKey(K targetKey);
    
    }

 public K parent(K key) {
        if(key == null) {
            throw new IllegalArgumentException();
        }
        return parent(key,  null);
    }

    public K parent(K key,K root)
    {
        K returnKey = null;

        if(key.compareTo(this.key) < 0) {
            returnKey =  left.parent(key, this.key);
        }
         else if (key.compareTo(this.key) > 0) {
            returnKey =  right.parent(key, this.key);
        } else {
            returnKey = root;
        }
        return returnKey;
    }