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 Python3 (judge verdict: Accepted)

class D_Linked_List:
    """Doubly Linked List class.

    ?????¢?????´???????????¨??????????§???????p.96-100.
    """
    class Node:
        def __init__(self, key, next = None, prev = None):
            self.next = next
            self.prev = prev
            self.key = key

    def __init__(self):
        self.nil = D_Linked_List.Node(None)
        self.nil.next = self.nil
        self.nil.prev = self.nil

    def insert(self, key):
        node_x = D_Linked_List.Node(key, self.nil.next, self.nil)
        self.nil.next.prev = node_x
        self.nil.next = node_x

    def _listSearch(self, key):
        cur_node = self.nil.next
        while (cur_node != self.nil) and (cur_node.key != key):
            cur_node = cur_node.next
        return cur_node

    def _deleteNode(self, node):
        if node == self.nil:
            return None
        node.prev.next = node.next
        node.next.prev = node.prev

    def deleteFirst(self):
        self._deleteNode(self.nil.next)

    def deleteLast(self):
        self._deleteNode(self.nil.prev)

    def deleteKey(self, key):
        node = self._listSearch(key)
        self._deleteNode(node)

    def show_keys(self):    # ????????????node???key????????????????????§??????????????????????????§???
        cur_node = self.nil.next
        keys = []
        while cur_node != self.nil:
            keys.append(cur_node.key)
            cur_node = cur_node.next
        print(' '.join(keys))


import sys

d_ll = D_Linked_List()

for i in sys.stdin:
    if 'insert' in i:
        x = i[7:-1]
        d_ll.insert(x)
    elif 'deleteFirst' in i:
        d_ll.deleteFirst()
    elif 'deleteLast' in i:
        d_ll.deleteLast()
    elif 'delete' in i:
        x = i[7:-1]
        d_ll.deleteKey(x)
    else:   # ??\??????????????¨?????????command??§??????????????§???pass?????????
        pass

d_ll.show_keys()