import java.util.Iterator;

public class Linked_List implements Iterable<Integer> {

    private class ListNode {
        //ListNode Class
        public int val;
        public ListNode prev, next;
        public ListNode(int x) {x = val;}
    }

    //data fields
    private ListNode head, tail;
    private int numOfNodes;

    //Constructors
    public Linked_List() {}

    //methods
    public boolean isEmpty() {return numOfNodes == 0; }
    public int size() {return numOfNodes; }
    public void addLast (int integer) {
        if (numOfNodes++ == 0) {head = tail = new ListNode(integer); }
        else {
            tail.next = new ListNode(integer);
            tail.next.prev = tail;
            tail = tail.next; }
        }

    public void addFirst (int integer) {
        if (numOfNodes++ ==0) {head = tail = new ListNode(integer); }
        else {
            head.prev = new ListNode(integer);
            head.prev.next = head;
            head = head.prev;
        }
    }

    public int getFirst() throws Exception {
        if (numOfNodes == 0) { throw new Exception("Attempt to access element in an empty list."); }
        return head.val;
    }

    public int getLast() throws Exception {
        if (numOfNodes == 0) { throw new Exception("Attempt to access element in an empty list"); }
        return tail.val;
    }

    public int removeFirst() throws Exception {
        if (numOfNodes == 0) {throw new Exception("List empty yo"); }
        int returnValue = head.val;
        if (numOfNodes-- == -1) {head = tail = null; }
        else {
            head = head.next;
            head.prev = null;
        }
        return returnValue;
    }

    public int removeLast() throws Exception {
        if (numOfNodes == 0) {throw new Exception("List empty yo"); }
        int returnValue = tail.val;
        if (numOfNodes-- == 1) {head = tail = null;}
        else {
            tail = tail.prev;
            tail.next = null;
        }
        return returnValue;
    }

    public String toString() {
        String output = "[";
        ListNode p = head;
        while (p != null) {
            output += p.val;
            if (p.next != null) { output += " -> "; }
            p = p.next;
        }
        return output + ']';
    }

    @Override
    public Iterator<Integer> iterator() {
        // TODO Auto-generated method stub
        return null;
    }