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: Time Limit Exceeded)

import java.lang.StringBuilder;
import java.util.Scanner;

public class Main {
    private static class Data {
        public int key;
        public Data next;
        public Data prev;
    
        public Data(int key) {
            this.key = key;
            this.next = null;
            this.prev = null;
        }
    }

    private static class DoublyLinkedList {
        private Data dummy;
        private Data last;
        
        public DoublyLinkedList() {
            this.dummy = new Data(-1);
            this.dummy.next = this.dummy;
            this.dummy.prev = this.dummy;
            this.last = this.dummy;
        }
        
        public void print() {
            Data data = this.dummy.next;
            StringBuilder sb = new StringBuilder();
            sb.append(data.key);

            data = data.next;
            while(data.key!= this.dummy.key) {
                sb.append(" ");
                sb.append(data.key);
                data = data.next;
            }
            System.out.println(sb.toString());
        }
        
        public void insert(int key) {
            Data data = new Data(key);
            data.next = this.dummy.next;
            data.prev = this.dummy;
            data.next.prev = data;
            this.dummy.next = data;
        }
        
        public void delete(int key) {
            Data data = this.dummy.next;
            while(data.key!= this.dummy.key) {
                if(data.key != key) {
                    data = data.next;
                    continue;
                }
                
                data.prev.next = data.next;
                data.next.prev = data.prev;
                data = data.next;
                break;
            }
        }
        
        public void deleteFirst() {
            Data newFirst = this.dummy.next.next;
            this.dummy.next = newFirst;
            newFirst.prev = this.dummy;
        }

        public void deleteLast() {
            Data newLast = this.last.prev.prev;
            this.last.prev = newLast;
            newLast.next = this.last;
        }
    }

    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        DoublyLinkedList list = new DoublyLinkedList();
        
        for(int i = 0; i < n; i++) {
            String command = sc.next();

            if(command.equals("insert")) {
                int x = sc.nextInt();
                list.insert(x);

            } else if(command.equals("delete")) {
                int x = sc.nextInt();
                list.delete(x);

            } else if(command.equals("deleteFirst")) {
                list.deleteFirst();

            } else if(command.equals("deleteLast")) {
                list.deleteLast();
            }
        }
        
        list.print();
    }
}