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>

typedef struct node{
	int num;
	struct node *prev;
	struct node *next;
} List;

List *list;

List *insert(List *head, int x)
{
	List *p;
	if((p=(List *)malloc(sizeof(List))) == NULL){
		printf("malloc error\n");
		exit(1);
	}
	p->num = x;
	p->next=head;
	if(head != NULL){
		p->prev = head->prev;
		head->prev = p;
	}//????´?1??\?????????????°?????????????????§?????????????
	else p->prev=p;//????´?0?????¨???????°????????????????
	return p;
}

List *deleteFirst(List *head)
{
	List *p;
	if(head==NULL) return head;
	if(head->next == NULL){
		free(head); return NULL;
	}
	p=head->next; p->prev=head->prev;//????°?????????????????§??????????
	free(head); return p;
}

List *deleteLast(List *head)
{
	if(head==NULL) return head;
	if(head->next == NULL){
		free(head); return NULL;
	}
	else{ 
		head->prev=head->prev->prev;
		free(head->prev->next);
		head->prev->next=NULL;
		return head;
	}
}
List *delete_x(List *head,int x){
	List *p=head->next;
	if(head->num == x){
		head = deleteFirst(head);
		return head;
	}
	while(p != NULL){
		if(p->num == x){
			p->prev->next = p->next;
			if(p->next != NULL) p->next->prev = p->prev;
			else head->prev = p->prev;
			free(p);
			break;
		}
		p = p->next;
	}
	return head;
}

void showAll(List *head){
	List *p=head;
	
	if(p != NULL){
		printf("%d", p->num);
		p = p->next;
	}
	while( p!= NULL){
		printf(" %d", p->num); p=p->next;
	}
	printf("\n");
}


int main(void){
	char com[13];
	int x,n;
	int i;
	list=NULL;
	scanf("%d",&n);
	for(i=0;i<n;i++){
		scanf("%s", com);
		if(strcmp( com, "insert") == 0){
			if(scanf("%d", &x) ==  -1){
				printf("No number\n");
				exit(1);
			}
			list=insert(list,x);
		}
		else if(strcmp( com, "delete") == 0){
			if(scanf("%d", &x) ==  -1){
				printf("No number\n");
				exit(1);
			}
			list=delete_x(list,x);
		}
		else if(strcmp( com, "deleteFirst") == 0){
			list=deleteFirst(list);
		}
		else if(strcmp( com, "deleteLast") == 0){
			list=deleteLast(list);
		}
	}
	showAll(list);
	return 0;
}