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,*prev;
};

struct Node *nil;

struct Node* listSearch(int key){
    struct Node *cur = nil->next; //番兵の次の要素からたどる
    while( cur != nil && cur->key != key){
        cur=cur->next;
    }
    return cur; //cur は現在位置のノードポインタ
}

void input(){ //リストの初期化
    nil = (struct Node *)malloc(sizeof(struct Node));
    nil->next=nil;
    nil->prev=nil;
}

void print(){
    struct Node *cur = nil->next;
    int i = 0;
    while(1){
        if( cur == nil) break;
        if( i++>0 ) printf(" ");
        printf("%d",cur->key);
        cur = cur->next;
    }
    printf("\n");
}

void deleteNode(struct Node *t){
    if(t==nil) return; //tが番兵と同値なら処理しない
    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){//検索したノードを除去
    deleteNode(listSearch(key));
}

void insert(int key){
    struct Node *x = (struct Node *)malloc(sizeof(struct Node));
    x->key = key; //番兵の直後に要素を追加
    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);
    
    input();
    
    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--;
        }
    }
    
    print();
    
    return 0;
    
}