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 CMD 16

typedef struct node_t{
  long key;
  struct node_t *prev;
  struct node_t *next;
} node; 

node *head, *tail;

int insert(node *add){
  if (!add)
    return -1;
  else if (!head)
    // when add a first node
    head = tail = add;
  else {
    head->prev = add;
    add->next = head;
    head = add;
  }
  return 0;
}

node *search(long searchKey){
  node *tmp = head;
  while (tmp) {
    if (tmp->key == searchKey)
      return tmp;
    tmp = tmp->next;
  }
  return NULL;
}

int delete(long deleteKey){
  node *del;
  if (!(del = search(deleteKey)))
    return -1;
  else {
    if (del == head) {
      head = head->next;
      if (head)
    	head->prev = NULL;
      else
	// list is empty
    	head = tail = NULL;
    }
    else if (del == tail) {
      tail = tail->prev;
      tail->next = NULL;
    }
    else {
      // delete node is middle in the list
      del->prev->next = del->next;
      del->next->prev = del->prev;
    }
    free(del);
    return 0;
  }
  return 1;
}

int deleteFirst(){
  if (head == tail && tail == NULL)
    return -1;
  else {
    head = head->next;
    if (head)
      head->prev = NULL;
    else
      // list is empty
      head = tail = NULL;
    return 0;
  }
}

int deleteLast(){
  if (head == tail && tail == NULL)
    return -1;
  else {
    tail = tail->prev;
    if (tail)
      tail->next = NULL;
    else
      // list is empty
      head = tail = NULL;
    return 0;
  }
}

int main(void){
  long n, i, key;
  char command[CMD] = {0};
  char ins[] = "insert", del[] = "delete";
  char delF[] = "deleteFirst", delL[] = "deleteLast";
  node *tmp;
  scanf("%ld", &n);
  for (i = 0; i < n; i++) {
    scanf("%s %ld", command, &key);
    if (!(strcmp(command, ins))) {
      tmp = (node *)malloc(sizeof(node));
      tmp->key = key;
      tmp->next = NULL;
      tmp->prev = NULL;
      insert(tmp);
    }
    else if(!(strcmp(command, del)))
      delete(key);
    else if(!(strcmp(command, delF)))
      deleteFirst();
    else if(!(strcmp(command, delL)))
      deleteLast();
    else
      puts("Not available command!");
  }
  
  // output
  char *sp = "";
  for (tmp = head; tmp != NULL; tmp = tmp->next) {
    printf("%s%ld", sp, tmp->key);
    sp = " ";
  }
  puts("");
  return 0;
}