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 <cstdio>
#include <cstdlib>
#include <cstring>

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

Node *nil;

Node* listSearch(int key){
    Node *cur = nil ->next;         //从头结点后面的元素开始访问
    while (cur != nil && cur ->key != key){
        cur = cur -> next;
    }
    return cur;
}


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

void printList(){
    Node *cur = nil->next;
    int isf = 0;
    while(true){
        if(cur == nil) break;
        if(isf++ > 0) printf(" ");
        printf("%d", cur->key);
        cur = cur ->next;
    }
    printf("\n");
}


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 insert(int key){
    Node* X = (Node*)malloc(sizeof(Node));
    X->key = key;
    X->next = nil->next;
    nil->next->prev = X;
    nil->next = X;
    X->prev = nil;
}


int main(){
    int key, n, i;
    int size = 0;
    char com[20];
    int np = 0, nd = 0;
    scanf("%d",&n);
    init();
    for(i = 0; i<n; i++){
        scanf("%s%d",com,&key);
        if(com[0]=='i') {insert(key); np++; size++;}
        else if(com[0] == 'd'){
            if(strlen(com) > 6){
                if(com[6] == 'F') deleteFirst();
                else if(com[6] =='L') deleteLast();
            }else{
                deleteKey(key); nd++;
            }
            size--;
        }
    }

    printList();
    return 0;
}