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++11 (judge verdict: Accepted)

#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>

using namespace std;

class linkedlist
{
public:
	int data;
	linkedlist* next;
	linkedlist* back;
	linkedlist(int n)
	{
		data = n;
		next = nullptr;
		back = nullptr;
	}
	linkedlist(int n, linkedlist* b)
	{
		data = n;
		next = nullptr;
		back = b;
	}
	~linkedlist()
	{
		if (back && next)
		{
			linkedlist* t = this->back;
			this->back->next = this->next;
			this->next->back = t;
			return;
		}
		if (back && next == nullptr)
		{
			this->back->next = nullptr;
			return;
		}
		if (back == nullptr && next)
		{
			this->next->back = nullptr;
		}
	}
};

class Llist
{
public:
	linkedlist* now;
	linkedlist* last;
	Llist()
	{
		now = nullptr;
		last = nullptr;
	}
	~Llist()
	{
		//arekaku
	}
	void _insert(int n)
	{
		linkedlist* t = new	linkedlist(n);
		if (now == nullptr)
		{
			now = t;
			last = t;
		}
		else
		{
			t->back = now;
			now->next = t;
			now = t;
		}
	}
	void _delete(int n)
	{
		linkedlist* t = now;
		while (t)
		{
			if (t->data != n)
			{
				t = t->back;
				continue;
			}
			else
			{
				if (t == now)
				{
					now = (now->back) ? now->back : nullptr;
				}
				if (t == last)
				{
					last = (last->next) ? last->next : nullptr;
				}
				delete t;
				break;
			}
		}
	}
	void _deleteFirst()
	{
		if (!now->back)
		{
			delete now;
			now = nullptr;
			last = nullptr;
			return;
		}
		linkedlist* t = now->back;
		delete now;
		now = t;
	}
	void _deleteLast()
	{
		if (!now->back)
		{
			delete now;
			now = nullptr;
			last = nullptr;
			return;
		}

		if (last->next)
		{
			last = last->next;
			delete last->back;
		}
		else
		{
			delete last;
			last = nullptr;
			now = nullptr;
		}
	}
};


int main()
{
	//ios::sync_with_stdio(false);
	Llist* l = new Llist();
	int n;
	scanf("%d",&n);
	for (int i = 0; i < n; i++)
	{
		string s;
		int t;
		cin >> s;
		//scanf("%s", s);
		if (s == "deleteFirst")
		{
			l->_deleteFirst();
			continue;
		}
		if (s == "deleteLast")
		{
			l->_deleteLast();
			continue;
		}
		scanf("%d", &t);
		if (s == "insert")
		{
			l->_insert(t);
			continue;
		}
		if (s == "delete")
		{
			l->_delete(t);
			continue;
		}

	}

	linkedlist* t = l->now;
	while (t)
	{
		//cout << t->data;
		printf("%d", t->data);
		if (t->back == nullptr)
		{
			printf("\n");
		}
		else
		{
			printf(" ");
		}
		t = t->back;
	}
	delete l;
	return 0;
}