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>

struct node{
  int key;
  struct node *next;
  struct node *prev;
};

typedef struct node * NodePointer;

NodePointer nil;
void insert(int key);
int delete(int key);
int deleteFirst();
int deleteLast();
void init();     /* This fanction is initializing nil. */

main(){
  int n, i, keydata;
  char com[12];
  NodePointer p;
  scanf("%d",&n);
  nil=malloc(sizeof(struct node));
  init();

  for (i=0;i<n;i++){
    scanf("%s",com);
    if(strlen(com)>6){
      if(com[6]=='F') {
        deleteFirst();
      }else{
        deleteLast();
      }
    }else{
      scanf("%d",&keydata);
      if(com[0]=='i'){
        insert(keydata);
      }else{
        delete(keydata);
      }
    }
  }

  for(p=nil->next;p!=nil->prev;p=p->next){
    printf("%d ",p->key);
  }
  printf("%d\n",p->key);
  free(nil);
  return 0;
}

void insert(int keydata){
  NodePointer x;
  x=malloc(sizeof(struct node));
  x->key=keydata;
  x->next=nil->next;
  x->prev=nil;
  nil->next=x;
  x->next->prev=x;
}

int delete(int keydata){
  NodePointer p;
  p=nil->next;
  while(p!=nil&&p->key!=keydata){
    p=p->next; /* search */
  }
  if(p==nil){  /* If not found, this function is forced termination.  */
    return 0;
  }else{
    p->prev->next=p->next;
    p->next->prev=p->prev;
    free(p);
    return 0;
  }
}

int deleteFirst(){
  NodePointer p;
  p=nil->next;
  nil->next=p->next;
  p->next->prev=nil;
  free(p);
  return 0;
}

int deleteLast(){
  NodePointer p;
  p=nil->prev;
  p->prev->next=nil;
  nil->prev=p->prev;
  free(p);
  return 0;
}

void init(){
  nil->next=nil;
  nil->prev=nil;
}