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


typedef struct node {
	int key;
	struct node *next, *prev;
} Node;

Node *nil;

Node* listSearch(int key) {
	Node *cur = nil->next;
	while (cur != nil && cur->key != key) {
		cur = cur->next;
	}
	return cur;
}

void init() {
	nil = (Node *)malloc(sizeof(Node));
	nil->next = nil;
	nil->prev = nil;
}

void printList() {
	Node *cur = nil->next;
	int isf = 0;
	while (1) {
		if (cur == nil)break;
		if (isf++ > 0)printf(" ");
		printf("%d", cur->key);
		cur = cur->next;
	}
	printf("\n");
}

void deleteNode(Node *t) {
	if (t == nil)return;
	t->prev->next = t->next;
	t->next->prev = t->prev;
	free(t);
}

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

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

void deleteKey(int key) {
	deleteNode(listSearch(key));
}

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

int main() {
	int key, n, i;
	int size = 0;
	char com[20];
	int np = 0, nd = 0;
	scanf("%d", &n);
	init();
	for (i = 0; i < n; i++) {
		scanf("%s%d", com, &key);
		if (com[0] == 'i') {
			insert(key);
			np++;
			nd++;
		} else if (com[0] == 'd') {
			if (strlen(com) > 6) {
				if (com[6] == 'F')deleteFirst();
				else if (com[6] == 'L')deleteLast();
			} else {
				deleteKey(key); nd++;
			}
			size--;
		}
	}
	printList();

	return 0;
}