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>

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

Node *nil;

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

void printList(){
  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");
}

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

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

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

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

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

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

int main(){
  int key,n,i,size=0,np=0,nd=0;
  char com[20];

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

  return 0;
}