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: Time Limit Exceeded)

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

struct list{
  long key;
  struct list *next;
  struct list *back;
};

void insert(long x, struct list *p);
void dele(long x,struct list *p);
void deleteFirst(struct list *p);
void deleteLast(struct list *p);
void show_list(struct list *p);
void free_list(struct list *p);
struct list *HEAD;
int main(void){
  int i,j;
  int n;
  char command[12];
  long x=0;
  struct list *head;

  head=(struct list *) malloc(sizeof(struct list));
  HEAD=head;
  head->next=head;
  head->back=head;
  head->key=0;
  scanf("%d",&n);
  for(i=0;i<n;i++){
    //printf("head=%d ",head->back->next->key);
    //printf("tail=%d\n",head->back->key);
    scanf("%s",command);
    if(strcmp(command,"insert")==0){
      scanf("%ld",&x);
      insert(x,head);
    }
    else if(strcmp(command,"delete")==0){
      scanf("%ld",&x);
      dele(x,head);
    }
    else if(strcmp(command,"deleteFirst")==0) deleteFirst(head);
    else deleteLast(head);
  }


  show_list(head);
  free_list(head);
  return 0;
}
void show_list(struct list *p){
  p=p->next;
  if(p!=HEAD){
    printf("%d",p->key);
    p=p->next;
  }
  while(p!=HEAD){
    printf(" %d",p->key);
    p=p->next;
  }
  printf("\n");
}
void free_list(struct list *p){
  struct list *p2;
  while(p!=HEAD){
    p2=p->next;
    free(p);
    p=p2;
  }
}
void insert(long x,struct list *p){
  struct list *p2;
  if((p2=(struct list *) malloc(sizeof(struct list))) == NULL){
      printf("malloc error\n");
  }
  p->next->back=p2;
  p2->key=x;
  p2->next=p->next;
  p2->back=p;
  p->next=p2;
  if(p->back==p) p->back=p2;

}
void dele(long x,struct list *p){
  while(p->next!=HEAD){
    if(p->next->key==x){
      free(p->next);
      p->next->next->back=p;
      p->next=p->next->next;
      break;
    }
    p=p->next;
  }

}
void deleteFirst(struct list *p){
  free(p->next);
  p->next->next->back=p->next;
  p->next=p->next->next;
}
void deleteLast(struct list *p){
  if(p->back!=HEAD){
      free(p->back);
      p->back->back->next=p;
      p->back=p->back->back;
  }
}