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* prev;
  struct Node* next;
};
typedef struct Node* NodePointer;
NodePointer head;
void init()
{
  head = (NodePointer)malloc(sizeof(struct Node));
  head->prev = head;
  head->next = head;
}
 
NodePointer search(int s)
{
  NodePointer x = head->next;
  while(x!=head && x->key != s) x = x->next;
  return x;
}
 
void insert(int m)
{
  NodePointer x;
  x=(NodePointer)malloc(sizeof(struct Node));
  x->key = m;
  x->next = head->next;
  head->next->prev = x;
  head->next = x;
  x->prev = head;
}
 
void delete(int del)
{
  NodePointer x = search(del);
  if(x == head) return;
  x->prev->next = x->next;
  x->next->prev = x->prev;
}
 
void deleteFirst(){
  NodePointer x = head->next;
  if(x == head) return;
  x->prev->next = x->next;
  x->next->prev = x->prev;
}
 
void deleteLast(){
  NodePointer x = head->prev;
  if(x == head) return;
  x->prev->next = x->next;
  x->next->prev = x->prev;
}
 
int main()
{
  int i,n,m;
  NodePointer j;
  char str[17];
  init();
  scanf("%d",&n);
  for(i=0; i<n; i++) {
    scanf("%s",str);
    if(!strcmp(str,"insert")){
      scanf("%d",&m);
      insert(m);
    }
    else if(!strcmp(str,"delete")){
      scanf("%d",&m);
      delete(m);
    }
    else if(!strcmp(str,"deleteFirst")){
      deleteFirst();
    }
    else if(!strcmp(str,"deleteLast")){
      deleteLast();
    }
  }
  
  for(j = head->next; j!=head;){
    printf("%d",j->key);
    if((j=j->next) != head) printf(" ");
  }
  printf("\n");
  
  return 0;
}