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<string.h>

int head,tail;
int Q[1000001]={0};

int insert(int *Q,int x)
{
	Q[head]=x;
	head++;
	return 0;
}
int delete(int *Q,int x)
{
	int i;
	for(i=head;i>=tail;i--)
	{
		if(Q[i]==x){Q[i]=-1;break;}
	}
	if(i==head){head--;}
	if(i==tail){tail++;}
	return 0;
}
int deleteFirst(int *Q)
{
	while(1)
	{
		if(Q[head-1]==-1){head--;}
		else{break;}
	}
	Q[head-1]=-1;
	head--;
	return 0;
}
int deleteLast(int *Q)
{
	while(1)
	{
		if(Q[tail]==-1){tail++;}
		else{break;}
	}
	Q[tail]=-1;
	tail++;
	return 0;
}
int main(void)
{
	int d=0,i,n,q,t;
	char s[16];
	scanf("%d",&n);
	head=tail=0;
	for(i=0;i<n;i++)
	{
		scanf("%s",s);
		if(!strcmp(s,"insert")){scanf("%d",&t);insert(Q,t);}
		else if(!strcmp(s,"delete")){scanf("%d",&t);delete(Q,t);}
		else if(!strcmp(s,"deleteFirst")){deleteFirst(Q);}
		else if(!strcmp(s,"deleteLast")){deleteLast(Q);}
	}
	for(i=head-1;i>=tail;i--)
	{
		if(Q[i]!=-1)
		{
			if(d==1){printf(" ");}
			printf("%d",Q[i]);
			d=1;
		}
	}
	printf("\n");

	return 0;
}