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

struct Node *nil;

struct Node* listSearch(int key){
  
  struct Node *cur = nil->next;
  while (cur != nil && cur->key != key){
    cur = cur->next;
  }
  return cur;
}

void init() {
  
  nil = (struct Node *)malloc(sizeof(struct Node));
  nil->next = nil;
  nil->prev = nil;
}

void printList() {
  
  struct Node *cur = nil->next;
  int isf = 0;
  
  while(1){
    if(cur == nil) break;
    if(isf++ > 0) printf(" ");
    printf("%d", cur->key);
    cur = cur->next;
  }
  printf("\n");
}

void deleteNode(struct Node *x){
  
  if(x == nil) return;
  x->prev->next = x->next;
  x->next->prev = x->prev;
  free(x);
}

void deleteFirst() {
  deleteNode(nil->next);
}

void deleteLast() {
  deleteNode(nil->prev);
}

void deleteKey(int key) {
  deleteNode(listSearch(key));
}

void insert(int key) {
  struct Node *x = (struct Node *)malloc(sizeof(struct Node));
  x->key = key;
  x->next = nil->next;
  nil->next->prev = x;
  nil->next = x;
  x->prev = nil;
}

int main() {

  int i;
  int n;
  int key;
  int np = 0;
  int nd = 0;
  int size = 0;
  char com[20];
  
  scanf("%d", &n);
  init();

  for(i=0; i<n; i++){
    scanf("%s", com);
    if(com[0] == 'i'){
      scanf("%d", &key);
      insert(key);
      np++;
      size++;
    }
    else if(com[0] == 'd'){
      if(strlen(com) > 6){
	if(com[6] == 'L') deleteLast();
	else if(com[6] == 'F') deleteFirst();
      }
      else{
	 scanf("%d", &key);
	deleteKey(key);
	nd++;
      }
      size--;
    }
  }

  printList();
  
  return 0;
}