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 C (judge verdict: Accepted)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct list{
  int key;
  struct list *next_ptr;
  struct list *prev_ptr;
};
struct list *nil;				/* 番兵 */
void init(){					/* リストの初期化 番兵の設置 */
  nil=malloc(sizeof(struct list));
  nil->next_ptr=nil;
  nil->prev_ptr=nil;
}
void insert(int key){
  struct list *new_list_ptr;
  new_list_ptr=malloc(sizeof(struct list));
  new_list_ptr->key=key;
  //番兵の直後に要素を追加
  new_list_ptr->next_ptr=nil->next_ptr;
  nil->next_ptr->prev_ptr=new_list_ptr;
  nil->next_ptr=new_list_ptr;
  new_list_ptr->prev_ptr=nil;
}
struct list *listsearch(int key){
  struct list *cur_ptr=nil->next_ptr;
  while((cur_ptr!=nil)&&(cur_ptr->key!=key)){
	cur_ptr=cur_ptr->next_ptr;
}
  return cur_ptr;
}
void delete(struct list *t){
  if(t==nil)
	return;
  t->next_ptr->prev_ptr=t->prev_ptr;
  t->prev_ptr->next_ptr=t->next_ptr;
  free(t);
}
void deleteFirst(){
  delete(nil->next_ptr);
}
void deleteLast(){
  delete(nil->prev_ptr);
}
void deleteKey(int key){
  delete(listsearch(key));
}
void printlist(){
  struct list *cur=nil->next_ptr;
  int isf=0;
  while(1){
	if(cur==nil)
	  break;
	if(isf>0)
	  printf(" ");
	++isf;
	printf("%d", cur->key);
	cur=cur->next_ptr;
}
  printf("\n");
}
int main(){
  int n;
  scanf("%d", &n);
  char line[20];
  int i;
  int key;
  init();
  for(i=0; i<n; ++i){
	scanf("%s %d", line, &key);
	if(strcmp(line, "insert")==0){
	  insert(key);
	}
	else if(strcmp(line, "delete")==0)
	  deleteKey(key);
	else if(strcmp(line, "deleteFirst")==0)
	  deleteFirst();
	else if(strcmp(line, "deleteLast")==0)
	  deleteLast();
  }
  printlist();
  return 0;
}