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 <ctype.h>
#include <string.h>
#include <malloc.h>
#include <stdlib.h>

typedef struct _node {
	int key;
	struct _node *next;
	struct _node *prev;
} NODE, *PNODE;

PNODE root;

void init(void) {
	root = (NODE *)malloc(sizeof(NODE));
	root->next = root;
	root->prev = root;
}

void insert(int key) {
	PNODE t = (NODE *)malloc(sizeof(NODE));
	t->key = key;
	t->next = root->next;
	t->prev = root;
	root->next->prev = t;
	root->next = t;
}

void deleteNode(PNODE node) {
	if (node == root) {
		return;
	}
	node->next->prev = node->prev;
	node->prev->next = node->next;
	free(node);
}

void _delete(int key) {
	PNODE cur = root->next;
	while (cur != root) {
		if (cur->key == key) {
			deleteNode(cur);
			return;
		}
		cur = cur->next;
	}
}

void deleteFirst(void) {
	deleteNode(root->next);
}

void deleteLast(void) {
	deleteNode(root->prev);
}

void printNodes(void) {
	PNODE cur = root->next;
	while (1) {
		printf("%d", cur->key);
		cur = cur->next;
		if (cur != root) {
			putchar(' ');
		}
		else {
			break;
		}
	}
	putchar('\n');
}

void release(void) {
	PNODE tmp, cur = root->next;
	while (cur != root) {
		tmp = cur;
		cur = cur->next;
		deleteNode(tmp);
	}
	free(root);
}

int main(void) {
	char cmd[100];
	int n, i, f;
	int key;

	init();

	scanf("%d", &n);

	f = 0;
	while (scanf("%s", cmd) > 0) {
		if (cmd[0] == 'i') {
			f = 1;
		}
		else if (cmd[0] == 'd') {
			switch (cmd[6]) {
			case '\0':
				f = 2;
				break;
			case 'F':
				deleteFirst();
				break;
			case 'L':
				deleteLast();
				break;
			}
		}
		else {
			key = atoi(cmd);
			if (f == 1) {
				insert(key);
			}
			else if (f == 2) {
				_delete(key);
			}
		}
	}

	printNodes();

	release();

	return 0;
}