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>
#include <math.h>

struct LIST{
	int x;
	struct LIST *pre,*next;
};

void show(struct LIST *L){
	L=L->next;
	while(1){
		if(L->next==NULL){
			break;
		}
		printf("%d",L->x);
		L=L->next;
		if(L->next==NULL){
			printf("\n");
			break;
		}else{
			printf(" ");
		}
	}
}

void insert(struct LIST *head,int x,struct LIST *tail){
	struct LIST *add=(struct LIST*)malloc(sizeof(struct LIST));
	
	head->next->pre=add;
	
	add->next=head->next;
	add->pre=head;
	add->x=x;

	head->next=add;
}

void Delete(struct LIST *L,int x,struct LIST *tail){
	L=L->next;
	while(1){
		if(L->x==x){
			L->next->pre=L->pre;
			L->pre->next=L->next;
			break;
		}
		L=L->next;
		if(L->x==tail->x){
			break;
		}
	}
}

void deleteFirst(struct LIST *head){
	head=head->next;
	head->next->pre=head->pre;
	head->pre->next=head->next;
}

void deleteLast(struct LIST *tail){
	tail=tail->pre;
	tail->next->pre=tail->pre;
	tail->pre->next=tail->next;
}

int main(){
	int n;
	char command[120];
	int x;
	struct LIST *head=(struct LIST*)malloc(sizeof(struct LIST));
	struct LIST *tail=(struct LIST*)malloc(sizeof(struct LIST));
	head->x=-1;
	head->pre=NULL;
	tail->x=-2;
	tail->next=NULL;
	head->next=tail;
	tail->pre=head;

	scanf("%d",&n);
	for(int i=0;i<n;i++){
		scanf("%s",command);
		if(strcmp(command,"insert")==0){
			scanf("%d",&x);
			insert(head,x,tail);
		}else if(strcmp(command,"delete")==0){
			scanf("%d",&x);
			Delete(head,x,tail);
		}else if(strcmp(command,"deleteFirst")==0){
			deleteFirst(head);
		}else{
			deleteLast(tail);
		}
		
	}
	show(head);	

   return 0;
}