import java.util.Optional;

public class InOrderSuccessorBinaryTree<K,V> extends SimpleBinaryTree<K,V> {
    public Optional<K> inOrderSuccessorKey(K key) {
        Optional <BinaryTreeNode <K, V>> node = Optional.ofNullable(root);
        Optional <K> successor = Optional.empty();
        while(node.isPresent() && !node.get().getKey().equals(key)) {
            if(((Comparable) node.get().getKey()).compareTo(key) > 0) {
                successor = node.map(BinaryTreeNode::getKey);
                node = node.flatMap(BinaryTreeNode::getLeft);
            } //end of if
            else
                node = node.flatMap(BinaryTreeNode::getRight);
        } //end of while
        return node.flatMap(BinaryTreeNode::getRight).map(this::minKey).map(Optional::of).orElse(successor);
    } //end of method inOrderSuccessorBinaryTreeKey
} //end of class