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 linklist *list;
struct linklist{
	int key;
	list next;
	list back;
};

list head;
list tail;

void init(void){
	head = (list)malloc(sizeof(*head));
	tail = head;
	tail->next = NULL;
}

void insert(int k){
	list x;
	x = (list)malloc(sizeof(*x));
	x->next = head;
	x->key = k;
	x->back = NULL;
	head->back = x;
	head = x;
//	printf("insert\n");
	return;
}

void delete(int k){
	list x;
	list ins;
	x = head;
//	printf("delete\n");
	if(head->key==k){
		head = head->next;
		head->back = NULL;
		free(x);
		return;
	}
	while(x->next!=tail){
		if((x->next)->key==k){
			ins = x->next;
			x->next = ins->next;
			ins->next->back = x;
			free(ins);
			return;
		}else{
			x = x->next;
		}
	}
	return;
}

void deleteFirst(void){
	list ins;
	ins = head;
	head = head->next;
	head->back = NULL;
	free(ins);
	return;
}

void deleteLast(void){
	list x;
	list ins;
	x = tail;
	tail = tail->back;
	tail->next = NULL;
	free(x);
	return;
}

int main(int argc, char argv[]){
	int n;
	char command[20] = "\0";
	int d;
	list x;

	init();

	scanf("%d",&n);

	int i,j;
	for(i=0;i<n;i++){
		scanf("%s %d",command,&d);
		if(!strcmp(command,"insert"))insert(d);
		else if(!strcmp(command,"delete"))delete(d);
		else if(!strcmp(command,"deleteFirst"))deleteFirst();
		else if(!strcmp(command,"deleteLast"))deleteLast();
		else printf("The command does not exist.\n");
//		printf("%s %d\n",command,d);

/*		j=0;
		x = head;
		if(x!=NULL)printf("%d",x->key);
		x = x->next;
		while(x != NULL&&j<22){
			printf(" ");
			printf("%d",x->key);
			x = x->next;
			j++;
		}
		printf("\n");
*/	}

	x = head;
	if(x!=tail)printf("%d",x->key);
	x = x->next;
	while(x != tail){
		printf(" ");
		printf("%d",x->key);
		x = x->next;
	}
	printf("\n");

	return 0;
}