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 <unistd.h>
#include <stdlib.h>

int main(){
  int head=1,tail=0,i,j,n,a[2000001],now;
  char c[20];
  scanf("%d",&n);
  //  for(i=0;i<n;i++)
  //a[i]=-1;
  for(i=0;i<n;i++){
    scanf("%s",c);
    if(c[0]=='i'){
      scanf("%d",&now);
      a[head] = now;
      head++;
    }else if(c[6]=='F'){
      a[head]=-1;
      while(a[head]==-1)
	head--;
      
    }else if(c[6]=='L'){
      a[tail]=-1;
      while(a[tail]==-1)
	tail++;
    }
    
    else{
      scanf("%d",&now);
      for(j=head-1;j>tail;j--){
	if(a[j]==now){
	  a[j]=-1;
	  break;
	}

      }

    }
    ;
    /*
    printf("tail%d head%d:",tail,head);
  for(j=head-1;j>tail;j--){
    if(a[j] != -1){
      printf("%d[%d]",a[j],j);
      j--;
      break;
    }
  }
  for(;j>tail;j--){
    if(a[j] != -1){
      printf(" %d[%d]",a[j],j);
    }
  }
  printf("\n");
    */
    ;

    ;/*
  for(j=head-1;j>tail;j--){
    if(a[j] != -1){
      printf("%d",a[j]);
      j--;
      break;
    }
  }
  for(;j>tail;j--){
    if(a[j] != -1){
      printf(" %d[%d]",a[j],j);
    }
  }
  printf("%s head=%d  ",c,head);
  printf("\n");
*/
    ;
    
    
  }

  for(j=head-1;j>tail;j--){
    if(a[j] != -1){
      printf("%d",a[j]);
      j--;
      break;
    }
  }
  for(;j>tail;j--){
    if(a[j] != -1){
      printf(" %d",a[j]);
    }
  }
  printf("\n");

}