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>

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

typedef struct node Node;

Node *nil;

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

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;
}

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

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(void){
  int n, i, a, b = 0;
  char s[100];
  scanf("%d",&n);
  init();
  for(i=0;i<n;i++){
    scanf("%s",s);
    if(s[0] == 'd'){
      if(s[6] == 'F') deleteFirst();
      else if(s[6] == 'L') deleteLast();
      else {
        scanf("%d",&a);
        deleteKey(a);
      }
    }
    else {
      scanf("%d",&a);
      insert(a);
      b++;
    }
  }
  printf("%d",nil->next->key);
  Node *c = nil->next->next;
  while(c != nil){
    printf(" %d",c->key);
    c = c->next;
  }
  printf("\n");
  return 0;
}