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>

typedef struct Node* NodePointer;

typedef struct Node {
  int key;
  NodePointer next, pre;
} Node;

NodePointer head, last;

NodePointer make_node(int key) {
  NodePointer tmp = malloc(sizeof(Node));
  tmp->next = tmp->pre = NULL;
  tmp->key = key;
  return tmp;
}

void init() {
  head = malloc(sizeof(Node));
  last = malloc(sizeof(Node));
  head->next = last;
  last->pre = head;
}

void insert(NodePointer s, int key) {
  NodePointer node = make_node(key);
  NodePointer t = s->next;
  s->next = node;
  node->pre = s;
  node->next = t;
  t->pre = node;
}

void delete(NodePointer p) {
  NodePointer s = p->pre;
  NodePointer t = p->next;
  s->next = t;
  t->pre = s;
  free(p);
}

int main() {
  int N, num;
  char str[30];
  NodePointer p;
  init();
  scanf(" %d ", &N);
  while(N--) {
    scanf(" %s ", str);
    if(!strcmp(str,"insert")) {
      scanf(" %d ", &num);
      insert(head,num);
    } else if(!strcmp(str,"delete")) {
      scanf(" %d ", &num);
      for(p = head->next; p != last; p = p->next) {
        if(p->key == num) {
          delete(p);
          break;
        }
      }
    } else if(!strcmp(str,"deleteFirst")) {
      delete(head->next);
    } else if(!strcmp(str,"deleteLast")) {
      delete(last->pre);
    }
  }

  for(p = head->next; p != last; p = p->next) {
    printf(p == head->next? "%d":" %d", p->key);
  }
  printf("\n");

  return 0;
}