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

typedef struct _cell cell;

cell head = { &head, &head, 0 };

void insert_first(int x) {
  // リストの先頭にキーxを持つ要素を追加する
  cell *new;
  new = (cell *)malloc(sizeof(cell));

  new->next = head.next;
  new->prev = &head;
  new->key = x;
  head.next->prev = new;
  head.next = new;
}

void delete(cell *x) {
  if(x == &head)
    return;

  // xのアドレスを持つ要素を削除する
  x->prev->next = x->next;
  x->next->prev = x->prev;

  free(x);
}

// キーxを持つ要素を探し，その要素を削除する
void search_and_delete(int x) {
  cell *p = head.next;

  while(p != &head && p->key != x)
    p = p->next;

  delete(p);
}

int main(void) {
  int n, i, key;
  char operation[20];
  cell *p;

  scanf("%d", &n);

  for(i = 0;i < n;i++) {
    scanf("%s %d", operation, &key);

    if(strstr(operation, "insert")) {
      insert_first(key);
    } else if(strstr(operation, "deleteFirst")) {
      delete(head.next);
    } else if(strstr(operation, "deleteLast")) {
      delete(head.prev);
    } else if(strstr(operation, "delete")) {
      search_and_delete(key);
    }
  }

  p = head.next;
  while(p != head.prev) {
    printf("%d ", p->key);
    p = p->next;
  }

  printf("%d\n", p->key);

  return 0;
}