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<stdlib.h>
#include<string.h>
#define CHECKPOINT printf("CHECKPOINT\n");
typedef struct list
{
  int key;
  struct list *prev;
  struct list *next;
} *listPointer;
listPointer nil;//head list
listPointer make1list(void);//just make 1 list
void insertkey(int);//insert with key integer
void insertlist(listPointer);//insert listPointer and reconnect
void deletekey(int);//delete key integer
void deleteFirst(void);//delete first one
void deleteLast(void);//delete last one
void deletelist(listPointer);//delete listPointer and reconnect
listPointer findlist(int);//find key integer
void printlist(void);//print linked list

int main(void)
{
  int i;//counter
  int n;//the number of operations
  int data;//new key
  char op[15];//operation
  nil=make1list();
  nil->prev=nil;
  nil->next=nil;
  scanf("%d",&n);
  for(i=0;i<n;i++)
    {
      scanf("%s%d",op,&data);
      if(strcmp(op,"insert")==0) insertkey(data);
      if(strcmp(op,"delete")==0) deletekey(data);
      if(strcmp(op,"deleteFirst")==0) deleteFirst();
      if(strcmp(op,"deleteLast")==0) deleteLast();
    }
  printlist();
  return 0;
}
listPointer make1list(void)
{
  listPointer p=malloc(sizeof(struct list));
  return p;
}
void insertkey(int data)
{
  listPointer p=make1list();
  p->key=data;
  insertlist(p);
}
void insertlist(listPointer p)
{
  p->next=nil->next;
  nil->next->prev=p;
  nil->next=p;
  p->prev=nil;
}
void deletelist(listPointer p)
{
  if(p!=nil)
    {
      p->prev->next=p->next;
      p->next->prev=p->prev;
      free(p);
    }
}
void deleteFirst(void)
{
  deletelist(nil->next);
}
void deleteLast(void)
{
  deletelist(nil->prev);
}
void deletekey(int data)
{
  listPointer p=findlist(data);
  deletelist(p);
}
listPointer findlist(int data)
{
  listPointer p;
  for(p=nil->next;p!=nil;p=p->next)
    {
      if(p->key==data) break;
    }
  return p;
}
void printlist(void)
{
  int flag=0;
  listPointer p;
  for(p=nil;p->next!=nil;p=p->next)
    {
      if(flag) printf(" ");
      else flag=1;
      printf("%d",p->next->key);
    }
  printf("\n");
}