Null pointer error

import java.util.*;
public class Main {private class Node {
int data;
Node next;

	Node(int data, Node next) {
		this.data = data;
		this.next = next;
	}
}

private Node head;
private Node tail;
private int size;

public Main() {
	this.head = null;
	this.tail = null;
	this.size = 0;
}

public Main(Node head, Node tail, int size) {
	this.head = head;
	this.tail = tail;
	this.size = size;
}

// O(1)
public int size() {
return this.size;
}

// O(1)
public boolean isEmpty() {
return this.size() == 0;
}

// O(1)
public int getFirst() throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty.”);
}

	return this.head.data;
}

// O(1)
public int getLast() throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty.”);
}

	return this.tail.data;
}

// O(N)
public int getAt(int idx) throws Exception {
Node temp = this.getNodeAt(idx);
return temp.data;
}

// O(N)
private Node getNodeAt(int idx) throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty”);
}

	if (idx < 0 || idx >= this.size()) {
		throw new Exception("Invalid arguments");
	}

	Node retVal = this.head;
	for (int i = 0; i < idx; i++) {
		retVal = retVal.next;
	}

	return retVal;
}

// O(1)
public void addFirst(int data) {
Node node = new Node(data, this.head);

	if (this.size() == 0) {
		this.head = node;
		this.tail = node;
	} else {
		this.head = node;
	}

	this.size++;
}

// O(1)
public void addLast(int data) {
Node node = new Node(data, null);

	if (this.size() == 0) {
		this.head = node;
		this.tail = node;
	} else {
		this.tail.next = node;
		this.tail = node;
	}

	this.size++;
}

// O(n)
public void addAt(int idx, int data) throws Exception {
if (idx < 0 || idx > this.size()) {
throw new Exception(“Invalid arguments”);
}

	if (idx == 0) {
		this.addFirst(data);
	} else if (idx == this.size()) {
		this.addLast(data);
	} else {
		Node nm1 = this.getNodeAt(idx - 1);
		Node n = nm1.next;

		Node node = new Node(data, n);
		nm1.next = node;

		this.size++;
	}
}

// O(1)
public int removeFirst() throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty”);
}

	int retVal = this.head.data;

	if (this.size() == 1) {
		this.head = null;
		this.tail = null;
	} else {
		this.head = this.head.next;
	}

	this.size--;
	return retVal;
}

// O(n)
public int removeLast() throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty”);
}

	int retVal = this.tail.data;

	if (this.size() == 1) {
		this.head = null;
		this.tail = null;
	} else {
		Node sm2 = this.getNodeAt(this.size() - 2);
		sm2.next = null;
		this.tail = sm2;
	}

	this.size--;
	return retVal;
}

// O(n)
public int removeAt(int idx) throws Exception {
if (this.isEmpty()) {
throw new Exception(“List is empty”);
}

	if (idx < 0 || idx >= this.size()) {
		throw new Exception("Invalid arguments");
	}

	if (idx == 0) {
		return this.removeFirst();
	} else if (idx == this.size() - 1) {
		return this.removeLast();
	} else {
		Node nm1 = this.getNodeAt(idx - 1);
		Node n = nm1.next;
		Node np1 = n.next;

		nm1.next = np1;
		this.size--;

		return n.data;
	}
}

// O(n)
public void display() {
Node node = this.head;

	while (node != null) {
		System.out.print(node.data + " ");
		node = node.next;
	}

	// System.out.println("END");
}

public static int intersection(Main list,Main list1){
Node t1=list.head;
Node t2=list1.head;
int delta=Math.abs(list.size-list1.size);
if(list.size>list1.size){
for(int i=0;i<delta;i++){
t1=t1.next;
}}
else{
for(int i=0;i<delta;i++){
t2=t2.next;
}
}
while(t1!=t2){
t1=t1.next;
t2=t2.next;
}
return t1.data;
}

public static void main(String[] args) throws Exception {

	Scanner scn = new Scanner(System.in);
	
	// int M = scn.nextInt();

	Main list = new Main();

Main list1 = new Main ();
Scanner sc = new Scanner (System.in);
int n=sc.nextInt();

	for(int k=0;k<n;k++)
	{
	    list.addLast(sc.nextInt());
	}
	int v=sc.nextInt();
	for(int k=0;k<v;k++)
	{
	    list1.addLast(sc.nextInt());
	}


	System.out.print(list.intersection(list,list1));
	
}

}