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>

typedef struct node_s node;

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

node *head;

void insert(int x){
  node *t=malloc(sizeof(node));
  t->key=x;
  t->next=head->next;
  t->prev=head;
  head->next->prev=t;
  head->next=t;
}
void delete(node *n){
  if (n==NULL)return;
  n->prev->next=n->next;
  n->next->prev=n->prev;
  free(n);
}

node *find(int key){
  node *p;
  for (p=head->next;p->key!=-1;p=p->next){
    if (p->key==key){
      return p;
    }
  }
  return NULL;
}

int main(){
  int i,n,x;
  char cmd[16];
  node *tp;
    
  head = malloc(sizeof(node));
  head->key=-1;
  head->prev=head;
  head->next=head;
  
  scanf("%d",&n);

  for (i=0;i<n;i++){
    if(i<n)break;
  }

  for (i=0;i<n;i++){
    scanf("%s", &cmd);
    if (cmd[0]=='i'){
      scanf("%d",&x);
      insert(x);
    }else if (cmd[0]=='d'&&cmd[6]=='\0'){
      scanf("%d",&x);
      delete(find(x));
    }else if (cmd[0]=='d'&&cmd[6]=='F'){
      delete(head->next);
    }else if (cmd[0]=='d'&&cmd[6]=='L'){
      delete(head->prev);
    }
  }
  for (tp=head->next;tp->next->key!=-1;tp=tp->next)
    printf("%d " ,tp->key);
  printf("%d\n" ,tp->key);
  
  return 0;
}