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: Memory Limit Exceeded)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO ?????????????????????????????????????????????
		Scanner sc = new Scanner(System.in);
		int n= sc.nextInt();
		sc.nextLine();

		DoublyLL ll = new DoublyLL();

		for (int i=0; i < n; i++) {
			String command = sc.nextLine();

			switch (command) {
			case "deleteFirst":
				ll.deleteFirst();
				continue;
			case "deleteLast":
				ll.deleteLast();
				continue;
			}

//			int x = Integer.parseInt(command.split(" ")[1]);

			switch (command.split(" ")[0]) {
			case "insert":
				ll.insert(Integer.parseInt(command.split(" ")[1]));
				continue;
			case "delete":
				ll.delete(Integer.parseInt(command.split(" ")[1]));
				continue;
			}
		}

		StringBuilder sb = new StringBuilder();

		DoublyLL.Element e = ll.fal.next;
		if ( e != ll.fal)
			sb.append(e.key);
		e = e.next;
		while (e != ll.fal) {
			sb.append(" " + e.key);
			e = e.next;
		}
		System.out.println(sb);
	}

}

class DoublyLL {

	class Element {
		Element previous;

		int key;

		Element next;

		Element(Element previous, int key, Element next) {
			this.key = key;
			this.next = next;
			this.previous = previous;
		}
	}

	// dammy first and last element
	// fal <-> element_1 <-> element_2 <-> ... <-> element_n <-> fal
	Element fal;

	Element newfirst;
	Element temp;

	// constructor
	DoublyLL() {
		fal = new Element(null, 0, null);
		fal.previous = fal;
		fal.next = fal;
	}

	// insert x
	void insert(int x) {
//		Element newfirst = new Element(fal, x, fal.next);
		newfirst = new Element(fal, x, fal.next);
		fal.next.previous = newfirst;
		fal.next = newfirst;
	}

	// delete x
	void delete(int x) {
//		Element temp = fal.next;
		temp = fal.next;

		while (temp != fal) {
			if (temp.key == x) {
				temp.previous.next = temp.next;
				temp.next.previous = temp.previous;
				break;
			}
			temp = temp.next;
		}

	}

	void deleteFirst() {
		// temp : first element of this list
		Element temp = fal.next;
		temp = fal.next;
		fal.next = temp.next;
		temp.next.previous = fal;
	}

	void deleteLast() {
		// temp : last element of this list
//		Element temp = fal.previous;
		temp = fal.previous;
		temp.previous.next = fal;
		fal.previous = temp.previous;
	}
}