Problem:
Doubly Linked List

Your task is to implement a double linked list.

Write a program which performs the following operations:

insert x: insert an element with key x into the front of the list.

delete x: delete the first element which has the key of x from the list. If there is not such element, you need not do anything.

deleteFirst: delete the first element from the list.

deleteLast: delete the last element from the list.

Input

The input is given in the following format:

n

command1

command2

...

commandn

In the first line, the number of operations n is given. In the following n lines, the above mentioned operations are given in the following format:

insert x

delete x

deleteFirst

deleteLast

Output

Print all the element (key) in the list after the given operations. Two consequtive keys should be separated by a single space.

Constraints

The number of operations ≤ 2,000,000

The number of delete operations ≤ 20

0 ≤ value of a key ≤ 109

The number of elements in the list does not exceed 106

For a delete, deleteFirst or deleteLast operation, there is at least one element in the list.

Sample Input 1

7
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5

Sample Output 1

6 1 2

Sample Input 2

9
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5
deleteFirst
deleteLast

Sample Output 2

1

Solution in JAVA (judge verdict: Runtime Error)

import java.util.Scanner;


public class Main{

	static Node head = null;

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int cnt = new Integer(s.nextLine()).intValue();
		for(int i=0 ; i<cnt ; i++) {
			String[] str = s.nextLine().split(" ");
			if("insert".equals(str[0])) {
				insert(new Integer(str[1]).intValue());
			}else if("delete".equals(str[0])) {
				delete(new Integer(str[1]).intValue());
			}else if("deleteFirst".equals(str[0])) {
				deleteFirst();
			}else if("deleteLast".equals(str[0])) {
				deleteLast();
			}
		}
		String str = "";
		while(head!=null){
			str += head.value + " ";
			head = head.nextNode;
		}
		System.out.println(str.trim());
	}

	public static void insert(int value) {
		if(head == null) {
			head = new Main().new Node(value);
			return;
		}
		Node current = new Main().new Node(value);
		head.previousNode = current;
		current.nextNode = head;
		head = current;
	}

	public static void delete(int value) {
		if(head == null) {
			return;
		}
		Node n = head;
		while(n != null) {
			if(n.value == value && n.nextNode!=null && n.previousNode!=null) {
				n.nextNode.previousNode = n.previousNode;
				n.previousNode.nextNode = n.nextNode;
				break;
			}else if(n.value==value && n.previousNode==null && n.nextNode!=null){
				deleteFirst();
				break;
			}

			else if(n.value==value && n.nextNode==null && n.previousNode!=null){
				deleteLast();
				break;
			}


			n = n.nextNode;
		}
	}

	public static void deleteFirst() {
		if(head == null)
			return;
		Node d = head.nextNode;
		head.nextNode = null;
		d.previousNode = null;
		head = d;
	}

	public static void deleteLast() {
		if(head == null)
			return;

		if(head.nextNode == null){
			head = null;
			return;
		}

		Node n = head;
		while(n.nextNode != null) {
			n = n.nextNode;
		}
		n.previousNode.nextNode = null;
	}

	public class Node {
		int value;
		Node previousNode;
		Node nextNode;

		public Node(int value) {
			this.value = value;
			this.previousNode = null;
			this.nextNode = null;
		}

		public void setPreviousNode(Node node) {
			this.previousNode = node;
		}

		public void setNextNode(Node node) {
			this.nextNode = node;
		}
	}
}