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;
    struct node *prev;
};
 
typedef struct node * NodePointer;
 
NodePointer nil;
 
/*
NodePointer listSearch(int key){
    NodePointer t;
    for(t = nil -> next;t != nil;t = t -> next){
        if(t -> key == key){
            return t;
        }
    }
}*/
 
void init(){
    nil = (struct node*)malloc(sizeof(struct node));
    nil -> next = nil;
    nil -> prev = nil;
}
 
void printList(){
    struct node *p;
    for(p = nil -> next; p != nil;p = p -> next){
        if(p != nil -> prev){
            printf("%d ", p -> key);
        }else if(p == nil -> prev){
            printf("%d\n", p -> key);
        }
    }
    /*
    NodePointer cur = nil -> next;
    int isf = 1;
    while(1){
        if(cur == nil) break;
        if(isf == 0) printf(" ");
        printf("%d\n", cur -> key);
        cur = cur -> next;
        isf = 0;
    }
    printf("\n");
    */
}
 
void deleteNode(NodePointer t){
    if(t -> prev == nil){//deleteFirst
        nil -> next = t -> next;
        t -> next -> prev = nil;
        free(t);
    }else if(t -> next == nil){//deleteLast
        nil -> prev = t -> prev;
        t -> prev -> next = nil;
        free(t);
    }else{
        t -> prev -> next = t -> next;
        t -> next -> prev = t -> prev;
        free(t);
    }
}
 
void deleteFirst(){
    NodePointer t = nil -> next;
    if(t == nil) return;
    deleteNode(t);
}
 
void deleteLast(){
    NodePointer t = nil -> prev;
    if(t == nil) return;
    deleteNode(t);
}
 
void del(int key){
    //NodePointer t;
    struct node *p;
 
    if(nil -> key == key){
        deleteFirst();
        return;
    }
    for(p = nil -> next;p != nil;p = p -> next){
        if(p -> key == key){
            deleteNode(p);
            return;
        }
    }
 
    //t = listSearch(t);
 
}
 
void insert(int key){
    NodePointer x;
    x = (struct node*)malloc(sizeof(struct node));
    x -> key = key;
 
    if(nil -> next == nil){
        nil -> prev = x;
    }
 
    x -> next = nil -> next;
    x -> prev = nil;
    nil -> next -> prev = x;
    nil -> next = x;
}
 
int main(void){
    int key,n,i;
    char com[200];
    scanf("%d", &n);
 
    init();
 
    for(i = 0;i < n;i++){
        scanf("%s", com);
        if(com[0] == 'i'){
            scanf("%d", &key);
            insert(key);
        }else if(com[0] == 'd'){
            if(strlen(com) > 6){
                if(com[6] == 'F') deleteFirst();
                else if(com[6] == 'L') deleteLast();
            }else{
                scanf("%d", &key);
                del(key);
            }
        }
    }
 
    printList();
    return 0;
}