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>

typedef struct Node {
  int x;
  struct Node *prev;
  struct Node *next;
} Node;

Node *head, *tail;

void dump() {
  int l = 0;
  Node *p = head;
  while (p != NULL) {
    if (l == 0)  printf("%d",  p->x);
    else printf(" %d", p->x);
    ++l;
    p = p->next;
  }
  printf("\n");
}

void insert(int x) {
  Node *nod = malloc(sizeof(Node));;
  nod->x = x;
  nod->prev = NULL;
  nod->next = NULL;

  if (head == NULL) {
    head = nod;
    tail = nod;
  } else {
    nod->next = head;
    head->prev = nod;
    head = nod;
  }
}

void delete(int x) {
  Node *p = head;
  int flg = 0;

  while (flg == 0 && p != NULL) {
    if (p->x == x) {
      Node *c = p;
      if (c->prev != NULL) {
        p->prev->next = c->next;
      } else {
        head = c->next;
      }

      if (c->next != NULL) {
        p->next->prev = c->prev;
      } else {
        tail = c->prev;
      }
      flg = 1;
    } else {
      p = p->next;
    }
  }
}

void deleteFirst() {
  if (head == tail) {
    head = NULL;
    tail = NULL;
  } else if (head != NULL) {
    if (head->next != NULL) head->next->prev = NULL;
    head = head->next;
  }
}

void deleteLast() {
  if (head == tail) {
    head = NULL;
    tail = NULL;
  } else if (tail != NULL) {
    if (tail->prev != NULL) tail->prev->next = NULL;
    tail = tail->prev;
  }
}

int main(int argc, char **argv) {

  int n;
  scanf("%d\n", &n);

  int i;
  int x;
  char buf[256];
  char tmp[256];
  for (i = 0; i < n; ++i) {
    fgets(buf, 256, stdin);

    if (buf[0] == 'i' || buf[6] == ' ') {
      sscanf(buf, "%s %d", tmp, &x);

      if (buf[0] == 'i') insert(x);
      else               delete(x);
    } else if (buf[6] == 'F') {
      deleteFirst();
    } else if (buf[6] == 'L') {
      deleteLast();
    }
  }

  dump();

  return 0;
}