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 key;
	struct node *n;
	struct node *p;
} Node;

Node *nil;

Node *search(int key) {
	Node *res = nil->n;
	while (res != nil && res->key != key) res = res->n;
	return res;
}

void init() {
	nil = (Node *)malloc(sizeof(Node));
	nil->n = nil;
	nil->p = nil;
}

void print() {
	Node *res = nil->n;
	int i;
	while (1) {
		if (res == nil) break;
		if (i++ > 0) printf(" ");
		printf("%d", res->key);
		res = res->n;
	}
	printf("\n");
}

void delete(Node *t) {
	if (t == nil) return;
	t->p->n = t->n;
	t->n->p = t->p;
	free(t);
}

void deleteFirst() {
	delete(nil->n);
}

void deleteLast() {
	delete(nil->p);
}

void deleteKey(int key) {
	delete(search(key));
}

void insert(int key) {
	Node *x = (Node *)malloc(sizeof(Node));
	x->key = key;
	x->n = nil->n;
	nil->n->p = x;
	nil->n = x;
	x->p = nil;
}

int main() {
	int i,n;
	int key;
	char com[20];

	scanf("%d", &n);

	init();
	for (i=0;i<n;i++) {
		scanf("%s %d",com,&key);
		if (com[0] == 'i') {
			insert(key);
		}
		else  {
			if (strlen(com) > 6) {
				if (com[6] == 'F') deleteFirst();
				else if (com[6] == 'L') deleteLast();
			}
			else {
				deleteKey(key);
			}
		}
	}

	print();

	return 0;
}