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++14 (judge verdict: Accepted)

#include<cstdio>
#include<cstdlib>//for including malloc.
#include<cstring>
#include<iostream>

struct Node{
    int key;
    Node *prev,*next;
};

Node *NIL;

void init(){//reset.
    NIL=(Node *)malloc(sizeof(Node));
    NIL->prev = NIL;
    NIL->next = NIL;
}
void insert(int key){
    Node *x = (Node *)malloc(sizeof(Node));
    x->key = key;
    //insert just after NIL.
    x->next = NIL->next;
    NIL->next->prev = x;
    //if x is the first Node, NIL's prev pointer will join to x.
    NIL->next = x;
    x->prev = NIL;
}
Node* listSearch(int key){
    Node *cur = NIL->next;//seaching begin with the next of NIL.
    while(cur != NIL && cur->key != key){
        cur = cur->next;
    }
    return cur;
}
void deleteNode(Node *t){
    if(t == NIL)return;
    t->prev->next = t->next;
    t->next->prev = t->prev;
    free(t);
}
void deleteFirst(){
    deleteNode(NIL->next);
}
void deleteLast(){
    deleteNode(NIL->prev);
}
void deleteKey(int key){
    deleteNode(listSearch(key));
}
void print(){
    Node *cur = NIL->next;
    int cnt = 0;
    while(cur != NIL){
        if(cnt++>0)printf(" ");
        printf("%d",cur->key);
        cur = cur->next; 
    }
    printf("\n");
}

int main(){
    init();
    int n;
    std::cin>>n;
    for(int i=0;i<n;i++){
        char order[20];
        scanf("%s",order);
        if(order[0] == 'i'){
            int key;scanf("%d",&key);
            insert(key);
        }else if(order[0] == 'd' && strlen(order)<7){
            int key;scanf("%d",&key);
            deleteKey(key);
        }else if(order[6] == 'F'){
            deleteFirst();
        }else{
            deleteLast();
        }
    }
    print();
    return 0;
}