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)

// Doubly Linked List
#include <stdio.h>
#include <stdlib.h>

#define MAX 2000000

struct Node{
  int key;
  struct Node *prev;
  struct Node *next;
};
struct Node *nil; // 双方向リストの先頭のノードを指す番兵

// 双方向リストの初期化 //
void init(){
  nil = (struct Node *)malloc(sizeof(struct Node));
  nil->next = nil;
  nil->prev = nil;
}

// 与えられたkeyを持つノードを探し、そのポインタを返す //
struct Node* listSearch(int key){
  struct Node *x;
  x = nil->next; // 番兵の次の要素からたどる
  while(x != nil && x->key != key){
    x = x->next;
  }
  return x;
}

// 双方向リストへの要素の挿入 //
void insert(int key){
  struct Node *x = (struct Node *)malloc(sizeof(struct Node)); // xを持つノードを生成
  x->key = key;
   // 番兵の直後に要素を追加 //
  x->next = nil->next;
  nil->next->prev = x;
  nil->next = x;
  x->prev = nil;
}

// 双方向リストの要素の削除 //
void delete(struct Node *x){
  if(x == nil)return; // xが番兵の場合
  x->prev->next = x->next;
  x->next->prev = x->prev;
  free(x);
}

// 先頭のノードを削除 //
void deleteFirst(){
  delete(nil->next);
}

// 双方向リストの末尾の要素を削除 //
void deleteLast(){
  delete(nil->prev);
}

int main(){
  int i, n, key;
  char com[20];
  struct Node *x;
  
  init(); // リストの初期化
  
  scanf("%d", &n);
  for(i=0; i < n; i++){
    //printf("%d\n", i);
    scanf("%s", com);
    if(com[0] == 'i'){
      scanf("%d", &key);
      insert(key);
    }
    else if(com[0] == 'd' && com[6] == 'F')deleteFirst();
    else if(com[0] == 'd' && com[6] == 'L')deleteLast();
    else{
      scanf("%d", &key);
      delete(listSearch(key));
    }
  }
  x = nil->next;
  while(x != nil){
    if(x->prev != nil)printf(" ");
    printf("%d", x->key);
    x = x->next;
  }
  printf("\n");
  return 0;
}