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 cur = nil->next; // 番兵の次の要素から辿る
    while(cur->key != key && cur != nil){
        cur = cur->next;
        
    }
    return cur;
}

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

void printList(){
    NodePointer cur = nil->next;
    int isf = 0;
    while(1){
        if (cur == nil) break;
        if (isf ++ > 0)printf(" ");
        printf("%d", cur->key);
        cur = cur->next;
       
    }
    printf("\n");
}
void deleteNode(NodePointer t){
    if(t==nil) return; // t が番兵の場合は処理しない
    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){
    // 検索したノードを削除
    deleteNode(listSearch(key));
}
void insert(int key){
   NodePointer x = 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);
    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;
    
}