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 N 2000000

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

struct node *head;
struct node *tail;

void insert(int key) {
  struct node *ptr;
  ptr = (struct node*)malloc(sizeof(struct node));
  ptr -> key = key;
  ptr -> next = head -> next;
  ptr -> prev = head;
  head -> next -> prev = ptr;
  head -> next = ptr;
  return;
}

void delete(int key) {
  struct node *ptr, *tmp;
  for (ptr = head; ptr -> next != NULL; ptr = ptr -> next) {
    if (ptr -> next -> key == key) {
      tmp = ptr -> next;
      ptr -> next -> next -> prev = ptr;
      ptr -> next = ptr -> next -> next;
      free(tmp);
      return;
    }
  }
  return;
}

void deleteFirst() {
  struct node *tmp = head -> next;
  head -> next -> next -> prev = head;
  head -> next = head -> next -> next;
  free(tmp);
  return;
}

void deleteLast() {
  struct node *tmp = tail -> prev;
  tail -> prev -> prev -> next = tail;
  tail -> prev = tail -> prev -> prev;
  free(tmp);
  return;
}

int main(){
  
  int i, n, a;
  char str[12];
  struct node *ptr;
  
  head = malloc(sizeof(struct node));
  tail = malloc(sizeof(struct node));
  head -> next = tail;
  head -> prev = NULL;
  tail -> prev = head;
  tail -> next = NULL;
  
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    scanf("%s", str);
    if (strcmp(str, "insert") == 0) {
      scanf("%d", &a);
      insert(a);
      continue;
    }
    if (strcmp(str, "delete") == 0) {
      scanf("%d", &a);
      delete(a);
      continue;
    }
    if (strcmp(str, "deleteFirst") == 0) {
      deleteFirst();
      continue;
    }
    if (strcmp(str, "deleteLast") == 0) {
      deleteLast();
      continue;
    }
  }
  
  for (ptr = head -> next; ptr -> next -> next != NULL; ptr = ptr -> next) {
    printf("%d ", ptr -> key);
  }
  printf("%d\n", ptr -> key);
  
  return 0;
}