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>
typedef struct Node{
  int key;
  struct Node *next;
  struct Node *prev;
}Node;

Node *sen;

Node* listsearch(int key){
  Node *cur=sen->next;
  while(cur!=sen && cur->key!=key){cur=cur->next;}
  return cur;
}

void init(){
  sen=(Node *)malloc(sizeof(Node));
  sen->next=sen;
  sen->prev=sen;
}

void listprint(){
  Node *cur=sen->next;
  int i=0;
  while(1){
    if(cur==sen)break;
    if(i>0)printf(" ");
    printf("%d",cur->key);
    cur=cur->next;
    i++;
  }
  printf("\n");
}

void deleteNode(Node *t){
  if(t==sen)return;
  t->prev->next=t->next;
  t->next->prev=t->prev;
  free(t);
}

void deletefirst(){
  deleteNode(sen->next);
}
void deletelast(){
  deleteNode(sen->prev);
}

void deletekey(int key){
  deleteNode(listsearch(key));
}

void insert(int key){
  Node *a=(Node *)malloc(sizeof(Node));
  a->key=key;
  a->next=sen->next;
  sen->next->prev=a;
  sen->next=a;
  a->prev=sen;
}

int main(){
  int key,n,i;
  int size=0;
  char com[20];
  int p=0,d=0;
  scanf("%d",&n);
  init();
  for(i=0;i<n;i++){
    scanf("%s%d",com,&key);
    if(com[0]=='i'){
      insert(key);
      p++;
      size++;
    }
    else if(com[0]=='d'){
      if(strlen(com)>6){
	if(com[6]=='F')deletefirst();
	else if(com[6]=='L')deletelast();
      }
      else{
	deletekey(key);
	d++;
      }
      size--;
    }
  }
  listprint();
  return 0;
}