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 <iostream>
#include <cstdio>
using namespace std;

typedef struct _List{
	_List* front;
	_List* rear;
	int data;
}List;

int main(void){
	std::ios_base::sync_with_stdio(false);
	int n;
	string s;
	int d;
	cin >> n;
	List* top;
	List* end;
	List a;
	a.front = NULL;
	a.rear = NULL;
	a.data = 0;
	top = &a;
	end = &a;

	for(int i=0; i<n; i++){
		cin >> s;
		if(s == "insert"){
			cin >> d;
			List* temp = new List;
			
			temp->data = d;
			temp->rear = top;
			temp->front = NULL;
			top->front = temp;
			top = temp;
		}else if(s == "delete"){
			cin >> d;
			List* t = top;

			while(t != NULL && t->rear != NULL){
				if(t->data == d){
					if(t->front != NULL)(t->front)->rear = (t->rear);
					else top = t->rear;
					if(t->rear != NULL)(t->rear)->front = (t->front);
					break;
				}
				t = t->rear;
			}
		}else if(s == "deleteFirst"){
			if(top->rear != NULL)(top->rear)->front = NULL;
			top = top->rear;
		}else if(s == "deleteLast"){
			List* t = end;

			t = t->front;
			t = t->front;
			if(t != NULL)t->rear = end;
			else top = end;
			end->front = t;
		}
	}

	List* t = top;
	while(t->rear != NULL){
		if(t != top) cout << " ";
		cout << t->data;
		t = t->rear;
	}
	cout << endl;

	return 0;
}