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>

int flag = 0;

typedef struct list{
    int key;
    struct list *prev;
    struct list *next;
}List;

List *nil;

void insert(int key){
    List *p = (List *)malloc(sizeof(List));
    p->key = key;
    p->prev = nil;
    p->next = nil->next;
    nil->next->prev = p;
    nil->next = p;
}

void delete(int key){
    List *p = nil;
    while(1){
        if(p->next->key == key){
            p->next->next->prev = p;
            p->next = p->next->next;
            flag--;
            return ;
        }
        if(p->next == nil)return;
        p = p->next;
    }
}

void deleteFirst(void){
    nil->next->next->prev = nil;
    nil->next = nil->next->next;
    return ;
}

void deleteLast(){
    List *p = nil->prev->prev;
    p->next->next->prev = p;
    p->next = p->next->next;
}

void init(void){
    nil = (List *)malloc(sizeof(List));
    nil->prev = nil;
    nil->next = nil;
    nil->key = -111111;
}

int main(){
    int nm, i, key;
    char wd[12];
    
    init();

    scanf("%d", &nm);
    for(i = 0; i < nm; i++){
        scanf("%s", wd);
        if(strcmp(wd, "insert") == 0){
            scanf("%d", &key);
            insert(key);
            flag++;
        } 
        else if(strcmp(wd, "delete") == 0){
            scanf("%d", &key);
            delete(key);
        }
        else if(strcmp(wd, "deleteFirst") == 0){
            deleteFirst();
            flag--;
        } 
        else if(strcmp(wd, "deleteLast") == 0){
            deleteLast();
            flag--;
        } 
    }

    for(i = 1; i <= flag-1; i++){
        printf("%d ", nil->next->key);
        nil = nil->next;
    }
    printf("%d\n", nil->next->key);

    return 0;
}