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 *prev;
  struct node *next;

};

typedef struct node Node;

Node *head;

void init()
{
  head = (Node*)malloc(sizeof(Node));
  head -> prev = head;
  head -> next = head;
}
//head -> next?????? ??¢???????????°????????¢????????????
Node *search(int n){
  Node *x = head -> next;
  while( x != head && x -> key != n) x = x -> next;
  return x;
}
 
void insert(int n)//n???head????¬??????????????????????????????????
{
  Node *x;
  x = (Node*)malloc(sizeof(Node));
  x -> key = n;
  x -> next = head -> next;
  head -> next -> prev = x;
  head -> next = x;
  x -> prev = head;
}
 
void delete(int n)//m?????¢????????????????????´??????delete
{
  Node *x = search(n);
  if(x == head) return;
  x -> prev -> next = x -> next;
  x -> next -> prev = x -> prev;
}
 
void deleteFirst(){//?????????delete
  Node *x = head -> next;
  if(x == head) return;
  x -> prev -> next = x -> next;
  x -> next -> prev = x -> prev;
}
 
void deleteLast(){//?????????delete
  Node *x = head -> prev;
  if(x == head) return;
  x -> prev -> next = x -> next;
  x -> next -> prev = x -> prev;
}
 
int main()
{
  int n, i, m;
  Node *node;
  char c[20];
   
  init();
  scanf("%d", &n);

  for( i = 0; i < n; i++) {

    scanf("%s", c);

    if( strcmp(c,"insert") == 0){
      scanf("%d", &m);
      insert(m);
    }
    else if( strcmp(c,"delete") == 0){
      scanf("%d",&m);
      delete(m);
    }
    else if( strcmp(c,"deleteFirst") == 0){
      deleteFirst();
    }
    else if( strcmp(c,"deleteLast") == 0){
      deleteLast();
    }
  }
  for(node = head -> next; node != head;){
    printf("%d", node -> key);
    if((node = node -> next) != head) printf(" ");
  }
  printf("\n");
  return 0;
}