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 element {
  int key;

  struct element *fp;
  struct element *bp;
};

struct element list;

void insert(int x);
void delete(int x);
void deleteFirst();
void deleteLast();

int main(int argc, char *argv[])
{
  int n, i;
  char cmd[11 + 3];
  int x;
  struct element *p;

  list.fp = &list;
  list.bp = &list;

  scanf("%d", &n);

  for (i = 0; i < n; i++) {
    scanf("%s", cmd);
    if (!strcmp(cmd, "insert")) {
      scanf("%d", &x);
      insert(x);
    } else if (!strcmp(cmd, "delete")) {
      scanf("%d", &x);
      delete(x);
    } else if (!strcmp(cmd, "deleteFirst")) deleteFirst();
    else if (!strcmp(cmd, "deleteLast")) deleteLast();
  }

  for (p = list.fp; p != &list; p = p->fp) {
    if (p->fp != &list) printf("%d ", p->key);
    else printf("%d\n", p->key);
  }

  return 0;
}

void insert(int x)
{
  struct element *p;

  p = (struct element *)malloc(sizeof(struct element));

  p->key = x;

  p->fp = list.fp;
  p->bp = &list;
  p->fp->bp = p;
  list.fp = p;
}

void delete(int x)
{
  struct element *p;
  int found = 0;

  for (p = list.fp; p != &list; p = p->fp) {
    if (p->key == x) {
      found = 1;
      break;
    }
  }

  if (found == 1) {
    p->bp->fp = p->fp;
    p->fp->bp = p->bp;
    p->fp = NULL;
    p->bp = NULL;

    free(p);
  }
}

void deleteFirst()
{
  struct element *p = list.fp;

  list.fp = p->fp;
  p->fp->bp = &list;
  p->fp = NULL;
  p->bp = NULL;

  free(p);
}

void deleteLast()
{
  struct element *p = list.bp;

  p->bp->fp = &list;
  list.bp = p->bp;
  p->fp = NULL;
  p->bp = NULL;

  free(p);
}