public class Program {
    public static void main(String[] args) {
        DoublyLinkedList list = new DoublyLinkedList();
        list.insert_at_end(30);
        list.insert_at_end(20);
        list.insert_at_end(10);

        list.insert_between(30, 20, 50);
        list.print();
    }

}

class DoublyLinkedList {
    class Node {
        int info;
        Node next; // sledbenik
        Node prev; // prethodnik
        public Node( int x) {
            info = x;
            next = null;
            prev = null;
        }
    }
    Node head = null;
    void insert_at_beggining(int x) {
        if(head == null) {
            head = new Node(x);
            return;
        }
        Node new_node = new Node(x);
        new_node.next = head;
        head.prev = new_node;
        head = new_node;
    }
    void insert_at_end(int x) {
        if(head == null) {
            head = new Node(x);
            return;
        }
        Node new_node = new Node(x);
        Node tmp = head;
        while(tmp.next != null) {
            tmp = tmp.next;
        }
        tmp.next = new_node;
        new_node.prev = tmp;
    }
    void insert_between(int x, int y, int new_value) {
        Node x_node = head;

        while(x_node.info != x) {
            x_node = x_node.next;
        }

        Node y_node = x_node.next;
        Node new_node = new Node(new_value);

        x_node.next = new_node;
        new_node.prev = x_node;

        new_node.next = y_node;
        y_node.prev = new_node;
    }

    void print() {
        Node tmp = head;
        while(tmp != null) {
            System.out.print(tmp.info + " --> ");
            if(tmp.next == null) {
                break;
            }
            tmp = tmp.next;
        }
        System.out.println();
        while(tmp != null) {
            System.out.print(tmp.info + " <-- ");
            if(tmp.prev == null) {
                break;
            }
            tmp = tmp.prev;
        }
        System.out.println();
    }

}