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>
typedef struct _node {
    int key;
    struct _node *next;
    struct _node *prev;
} Node;
typedef Node *Nodepointer;

Nodepointer make_1node(int key){
    Nodepointer ret = malloc(sizeof(Node));
    ret->key = key;
    ret->next = NULL;
    ret->prev = NULL;
    return ret;
}

void delete_node(Nodepointer target){
    if(target -> next != NULL){
        target -> next ->prev = target -> prev;
    }
    if(target -> prev != NULL){
        target -> prev -> next = target -> next;
    }
    free(target);
}

int main(){
    int i, n, key;
    char command[12];
    Nodepointer head, tail, data, temp;
    head = tail = NULL;
    scanf("%d", &n);
    for(i = 0; i < n; i++){
        scanf("%s%d", command, &key);
        if (strcmp(command, "insert") == 0){
            data = make_1node(key);
            // 要素なし
            if (head == NULL){
                tail = data;
            }
            else{
                data->next = head;
                head->prev = data;
            }
            head = data;

            // deleteがない場合はdelete処理をしなくて良いので
            // continueで戻す
            continue;
        }
        if (strcmp(command, "delete") == 0){
            for(data = head; data != NULL; data = data->next){
                if (data->key == key){
                    break;
                }
            }
            if(data == NULL){
                // 該当データがなければdelete処理をせず
                // continueで戻す
                continue;
            }
        }
        if (strcmp(command, "deleteFirst") == 0){
            data = head;
        }
        if (strcmp(command, "deleteLast") == 0){
            data = tail;
        }
        if(head == data){
            head = head->next;
        }
        if(tail == data){
            tail = tail->prev;
        }
        delete_node(data);
    }
    for(i = 0, data = head; data != NULL; i++, data = data -> next){
        if(i){
            printf(" ");
        }
        printf("%d", data->key);
        fflush(stdout);

    }
    printf("\n");
    for(data = head; data != NULL; data = head){
        head = data->next;
        delete_node(data);
    }
    return 0;
}