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>

void insert(int);
void delete(int);
void deleteFirst(void);
void deleteLast(void);


struct node{
  int x;
  struct node *next;
  struct node *prev;
};
typedef struct node* Pnode;

Pnode head;
Pnode tail;
struct node *nod;
int main(){
  int x,i,n;
  head=malloc(sizeof(struct node));
  tail=malloc(sizeof(struct node));
  char cmd[20];
  head->next=tail;
  head->prev=NULL;
  tail->prev=head;
  tail->next=NULL;
  scanf("%d",&n);
  //ここまで初期設定
  for(i=0;i<n;i++){
    scanf("%s",cmd);
    if(strcmp(cmd,"insert")==0)scanf("%d",&x),insert(x);
    if(strcmp(cmd,"delete")==0)scanf("%d",&x),delete(x);
    if(strcmp(cmd,"deleteFirst")==0)delete(head->next->x);//delete(fast number)とdeleteFirstは同じ意味である。
    if(strcmp(cmd,"deleteLast")==0)deleteLast();
  }
  for(nod=head->next;nod->next!=NULL;nod=nod->next){
    if(nod->next->next==NULL)printf("%d",nod->x);
    else printf("%d ",nod->x);
  }

  printf("\n");
  return 0;
}
void insert(int x){
  Pnode newnode;
  newnode=malloc(sizeof(struct node));
  newnode->next=head->next;
  newnode->prev=head;
  head->next=newnode;
  newnode->next->prev=newnode;
  newnode->x=x;
}
void delete(int x){
  for(nod=head;nod->next!=NULL;nod=nod->next){
    if(nod->next->x==x)break;
  }
  if(nod->next!=NULL){
  nod->next->next->prev=nod;
  nod->next=nod->next->next;
  }
}
void deleteFirst(){
  
}
void deleteLast(){
  tail=tail->prev;
  tail->next=NULL;
  //tailを一つ下げる
}