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: Runtime Error)

<?php

fscanf(STDIN, "%d", $n);

$list = new LinkedList();

for ($i = 0; $i < $n; $i++) {
    $line = trim(fgets(STDIN));
    $commands = preg_split('/[ ]/', $line);

    switch ($commands[0]) {
    case 'insert':
        $list->insert($commands[1]);
        break;

    case 'delete':
        $list->delete($commands[1]);
        break;

    case 'deleteFirst':
        $list->deleteFirst();
        break;

    case 'deleteLast':
        $list->deleteLast();
        break;
    }
}

echo join(' ', $list->values())."\n";

class LinkedList
{
    private $first;
    private $last;

    public function __construct()
    {
        $this->first = null;
        $this->last  = null;
    }

    public function insert($value)
    {
        $this->first = new ListItem($value, $this->first);

        if (null === $this->last) {
            $this->last = $this->first;
        }
    }

    public function delete($value)
    {
        $item = $this->first;

        while (null !== $item) {
            if ($item->is($value)) {
                break;
            }

            $item = $item->next();
        }

        if (null === $item) {
            return;
        }

        if ($item->isFirst()) {
            $this->first = $item->next();
        }

        if ($item->isLast()) {
            $this->last = $item->prev();
        }

        $item->delete();
    }

    public function deleteFirst()
    {
        $item = $this->first;
        if (null === $item) {
            return;
        }

        $this->first = $item->next();

        if ($item->isLast()) {
            $this->last = null;
        }

        $item->delete();
    }

    public function deleteLast()
    {
        $item = $this->last;
        if (null === $item) {
            return;
        }

        $this->last = $item->prev();

        if ($item->isFirst()) {
            $this->first = null;
        }

        $item->delete();
    }

    public function values()
    {
        if (null === $this->first) {
            return array();
        }

        $item = $this->first;

        $values = array();
        while (null !== $item) {
            $values[] = $item->value();

            $item = $item->next();
        }

        return $values;
    }
}

class ListItem
{
    private $value;

    private $prev;
    private $next;

    public function __construct($value, $next)
    {
        $this->value = $value;
        $this->next = $next;

        if (null !== $next) {
            $next->setPrev($this);
        }
    }

    public function setPrev($prev)
    {
        $this->prev = $prev;
    }

    public function setNext($next)
    {
        $this->next = $next;
    }

    public function value()
    {
        return $this->value;
    }

    public function is($value)
    {
        return $this->value == $value;
    }

    public function prev()
    {
        return $this->prev;
    }

    public function next()
    {
        return $this->next;
    }

    public function isFirst()
    {
        return $this->prev === null;
    }

    public function isLast()
    {
        return $this->next === null;
    }

    public function delete()
    {
        if (null !== $this->prev) {
            $this->prev->setNext($this->next);
        }

        if (null !== $this->next) {
            $this->next->setPrev($this->prev);
        }
    }
}