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>

struct Node{
    int key;
    struct Node *next;
    struct Node *pref;
};
struct Node *head;
struct Node *lastNode;


void MakeHead(){
    head = malloc(sizeof(struct Node));
    head->key = 0;
    head->next = NULL;
    head->pref = NULL;
    lastNode = head;
}


void Insert(int num){
    struct Node *node;

    node = malloc(sizeof(struct Node));
    node->key = num;
    if(head->next == NULL){
        lastNode = node;
    }

    node->next = head->next;
    node->pref = head;
    if(head->next!=NULL)head->next->pref = node;
    head->next = node;
}


void Delete(int num){
    struct Node *i;

    for(i=head->next; i!=NULL; i=i->next){
        if(num == i->key){
            if(i->next != NULL){
                i->pref->next = i->next;
                i->next->pref = i->pref;
            }else{
                lastNode = i->pref;
                i->pref->next = NULL;
            }
            free(i);
            break;
        }
    }
}


void DeleteFirst(){
    if(head->next==NULL)return;
    if(head->next == lastNode){
        free(head->next);
        head->next = NULL;
        lastNode = head;
    }else{
        head->next = head->next->next;
        free(head->next->pref);
        head->next->pref = head;
    }
}


void DeleteLast(){
    if(lastNode == head)return;
    lastNode = lastNode->pref;
    lastNode->next = NULL;
    free(lastNode->next);
}

void OutputList(){
    struct Node *i;

    for(i=head; i->next!=NULL; i=i->next){
        printf("%d",i->next->key);
        if(i->next->next!=NULL) printf(" ");
    }
    printf("\n");
}


void Order(char *input,int num){
    char *order;

    order = malloc(sizeof(char) * (strlen(input)+1));
    strcpy(order,input);

    if(!strcmp(order,"insert")) Insert(num);
    else if(!strcmp(order,"delete")) Delete(num);
    else if(!strcmp(order,"deleteFirst")) DeleteFirst();
    else if(!strcmp(order,"deleteLast")) DeleteLast();
    else{
        printf("Invalid Order!!\n");
        free(order);
        exit(1);
    }
    free(order);
}


int main(){
    int order,n,i,keyNum=0;
    char buf[15];

    MakeHead();

    scanf("%d",&n);
    for(i=1; i<=n; i++){
        scanf("%s",buf);
        if(buf[6]=='\0'){
            scanf("%d",&keyNum);
        }
        Order(buf,keyNum);
    }

    OutputList();
    return 0;
}