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{
  unsigned int key;
  struct node *next;
  struct node *prev;
};

typedef struct node * NodePointer;

NodePointer nil;

NodePointer listSearch(int key){
  NodePointer n;
  for(n = nil->next; n != nil && n->key != key; n = n->next);
  return n;
}

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

void printList(){
  NodePointer cur;
  int isf = 1;
  for(cur = nil->next;cur != nil; cur = cur->next){
    if(isf == 0) printf(" ");
    printf("%d", cur->key);
    isf = 0;
  }
  printf("\n");
}

void deleteNode(NodePointer t){
  NodePointer delnode;
  delnode = t;
  delnode->prev->next = delnode->next;
  delnode->next->prev = delnode->prev;
  free(delnode);
}

void deleteFirst(){
  NodePointer t = nil->next;
  if(t == nil) return;
  deleteNode(t);
}

void deleteLast(){
  NodePointer t = nil->prev;
  if(t == nil) return;
  deleteNode(t);
}

void delete(int key){
  NodePointer t;
  t = listSearch(key);
  if(t == nil) return;
  deleteNode(t);
}

void insert(int key){
  NodePointer newnode;
  newnode = malloc(sizeof(struct node));
  newnode->key = key;
  newnode->next = nil->next;
  nil->next->prev = newnode;
  nil->next = newnode;
  newnode->prev = nil;
}

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

  printList();
  return 0;
}