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 Python (judge verdict: WA: Presentation Error)

import sys

class Deque:
    class Cell2:
        def __init__(self, x, y = None, z = None):
            self.data = x
            self.next = y
            self.prev = z

    def __init__(self):
        head = Deque.Cell2(None)
        head.next = head        
        head.prev = head
        self.size = 0
        self.head = head

    def push_front(self, x):
        h = self.head
        q = h.next
        p = Deque.Cell2(x, q, h)
        h.next = p
        q.prev = p
        self.size += 1

    def pop_back(self):
        if self.size == 0: raise IndexError
        h = self.head
        q = h.prev
        p = q.prev
        p.next = h
        h.prev = p
        self.size -= 1
        return q.data

    def pop_front(self):
        if self.size == 0: raise IndexError
        h = self.head
        q = h.next
        p = q.next
        p.prev = h
        h.next = p
        self.size -= 1
        return q.data

    def delete(self,x):
        h=self.head.next
        while h.data!=x:
            h=h.next
            if h==self.head: raise IndexError
        q=h.next
        p=h.prev
        q.prev=p
        p.next=q
        self.size -= 1
        return

    def __str__(self):
        if self.size == 0: return
        buff = ''
        n = self.size
        cp = self.head.next
        while n > 1:
            buff += '%s, ' % cp.data
            cp = cp.next
            n -= 1
        buff += '%s' % cp.data
        return buff
    
A=Deque()
n=int(raw_input())
while n:
    s = (raw_input().split())
    if s[0] == 'insert':
        A.push_front(s[1])
    elif s[0] == 'delete':
        A.delete(s[1])
    elif s[0] == 'deleteFirst':
        A.pop_front()
    elif s[0] == 'deleteLast':
        A.pop_back()
    n -=1
    
print A
print