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 PHP (judge verdict: Wrong Answer)

<?php
class Node{
  public $key;
  public $prev;
  public $next;
  function __construct($key){
    $this->key=$key;
  }
}
class Dll{
  public $END;
  public $first;
  public $last;
  function __construct(){
    $this->END=new Node('end');
    $this->first=$this->END;
    $this->last=$this->END;
  }
  function insert($x){
    $node=new Node($x);
    if($this->first->key=='end'){
      $node->prev=$node->next=$this->END;
      $this->first=$this->last=$node;
    }
    else{
      $this->first->prev=$node;
      $node->next=$this->first;
      $this->first=$node;
    }
  }
  function delete($x){
    $node=$this->first;
    while($node->key!='end'){
      if($node->key==$x){
        if($node->prev->key!='end') $node->prev->next=$node->next;
        if($node->next->key!='end') $node->next->prev=$node->prev;
        else $this->last=$node->prev;
        break;
      }
      $node=$node->next;
    }
  }
  function deleteFirst(){
    $this->first=$this->first->next;
    $this->first->prev=$this->END;
  }
  function deleteLast(){
    $this->last=$this->last->prev;
    $this->last->next=$this->END;
  }
  function view(){
    $node=$this->first;
    if($node->key=='end') return;
    print $node->key;
    $node=$node->next;
    while($node->key!='end'){
      print ' '.$node->key;
      $node=$node->next;
    }
  }
}
$dll=new Dll();
$N=(int)fgets(STDIN);
for($i=0;$i<$N;$i++){
  $com=trim(fgets(STDIN));
  if($com[6]=='F'){$dll->deleteFirst();continue;}
  if($com[6]=='L'){$dll->deleteLast();continue;}
  list($str,$key)=explode(' ',$com);
  if($str[0]=='i') $dll->insert($key);
  else $dll->delete($key);
}
$dll->view();
?>