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

struct List {
  int key;
  struct List *before;
  struct List *next;
};

struct List l;

void insert(int key) {
  struct List *a = malloc(sizeof(struct List));
  a->key = key;
  a->before = &l;
  a->next = l.next;
  l.next = a;
  a->next->before = a;
}

void delete(int b) {
  struct List *c;
  c = l.next;
  while(c->key != -1) {
    if (c->key == b) {
      c->next->before = c->before;
      c->before->next = c->next;
      break;
    }  
    c = c->next;
  } 
  }

void delete_First() {
  l.next = l.next->next;
  l.next->before = &l;
}

void delete_Last() {
  l.before = l.before->before;
  l.before->next = &l;
}

int main(void) {
  int n;
  char command[12];
  int key;
  struct List *box;
  l.key = -1;
  l.before = &l;
  l.next = &l;

  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%s", command);
    if (strcmp(command, "deleteFirst") == 0)
      delete_First();
    else if (strcmp(command, "deleteLast") == 0)
      delete_Last();
    else {
      scanf("%d", &key);
      if (strcmp(command, "insert") == 0) 
	insert(key);
      else if (strcmp(command, "delete") == 0)
	delete(key);
      }
    }
 
  
  box = l.next;
  while(box->key != -1) {
    printf("%d", box->key);
    box = box->next;
    if (box->key != -1)
      printf(" ");
  }
  printf("\n");
  
  return 0;
}