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: Time Limit Exceeded)

#include <stdio.h>
#define N 1000000

typedef struct
{
  int key;
  int back;
  int next;
}dlist;

dlist list[N];
int head=-1;
int tail=-1;
int num=0;
int flag=0;

void insert(int x)
{
  int i;
  list[num].key = x;
  if(flag==0)
  {
    list[num].back = -1;
    list[num].next = -1;
    tail=num;
    flag=1;
  }
  else
  {
    list[head].back = num;
    list[num].next = head;
  }
    head = num;
    num++;
//    printf("insert OK\n");
/*    i=head;
    while(1)
    {
      if(i==tail)
      {
        printf("%d\n",list[i].key);
        break;
      }
      else
      {
        printf("%d ",list[i].key);
        i = list[i].next;
      }
    }
*/
}

void dF()
{
  if(head==tail) flag=0;
  else
  {
    head = list[head].next;
  }
}

void dL()
{
  if(head==tail) flag=0;
  else
  {
    tail = list[tail].back;
  }
}

void delete(int x)
{
  int i;
  i=head;
  while(1)
  {
    if(list[i].key==x)
    {
      if(i==head)
      {
        if(head==tail) flag=0;
        else
        {
          head = list[head].next;
        }
      }
      else if(i==tail)
      {
        if(head==tail) flag=0;
        else
        {
          tail = list[tail].back;
        }
      }
      else
      {
        list[list[i].back].next=list[i].next;
        list[list[i].next].back=list[i].back;
      }
//      printf("delete OK\n");
/*      i=head;
      while(1)
      {
        if(i==tail)
        {
          printf("%d\n",list[i].key);
          break;
        }
        else
        {
          printf("%d ",list[i].key);
          i = list[i].next;
        }
      }
*/
      break;
    }
    else
    {
      i=list[i].next;
    }
  }

}


int main(void)
{
  int n;
  int i;
  int key;
  char c[100];

  scanf("%d",&n);
  for(i=0;i<n;i++)
  {
//    printf("scan:%d\n",i);
    scanf("%s",c);
    if(c[0]=='i')
    {
      scanf("%d",&key);
      insert(key);
//      printf("insert:%d\n",key);
    }
    else
    {
      if(c[6]=='F') dF();
      else if(c[6]=='L') dL();
      else
      {
          scanf("%d",&key);
          delete(key);
//          printf("delete:%d\n",key);
      }
    }
  }
  i=head;
  while(1)
  {
    if(i==tail)
    {
      printf("%d\n",list[i].key);
      break;
    }
    else
    {
      printf("%d ",list[i].key);
      i = list[i].next;
    }
  }


  return 0;
}