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 <string.h>
#include <stdio.h>
#include <stdlib.h>

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

struct Node *nil;

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

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

struct Node* listSearch(int key) {
    struct Node *cur = nil->next;
    while(cur != nil && cur->key != key ) {
        cur = cur-> next;
    }
    return cur;
}

void deleteNode(struct 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 printList() {
    struct Node *cur;
    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");
}


int main(void) {
    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;
}