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>

#define DEF_ELM_MAX 2000000

typedef struct _CMD {
	char cmd[32];
	int key;
} CMD;

CMD A[DEF_ELM_MAX];

typedef	struct _Node {
	int key;
	struct _Node *prev;
	struct _Node *next;
} Node;

Node *nil;	//	??????

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

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

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 ??????????????´?????????????????????
	}
	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(void)
{
	int i;
	int N;
	CMD *p;
//
	scanf("%d", &N);
	for ( i = 0 ; i < N ; i++ ) {
		p = &A[i];
		scanf("%s", p->cmd);
		if ( p->cmd[0] == 'd' && (p->cmd[6] == 'F' || p->cmd[6] == 'L') ) {
		} else {
			scanf("%d", &p->key);
		}
	}

	init();

	for ( i = 0 ; i < N ; i++ ) {
		p = &A[i];
		if (p->cmd[0] == 'i') {
			insert(p->key);
		} else if (p->cmd[0] == 'd') {
			if (p->cmd[6] == 'F') {
				deleteFirst();
			} else if (p->cmd[6] == 'L') {
				deleteLast();
			} else {
				deleteKey(p->key);
			}
		}
	}
	printList();

	return 0;
}