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>

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

Node *nil;

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

//?????°??????????????\??°??´????????????
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;
}

void Delete( Node *t ) {
    if ( t == nil ) return; // ??´??????????????????
    t->prev->next = t->next;
    t->next->prev = t->prev;
    free( t );
}

void DeleteFirst() {
    Delete( nil->next );
}

void DeleteLast() {
    Delete( nil->prev );
}

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

void DeleteKey( int key ) {
    // ??????????´¢??°?????????
    Delete( ListSearch(key) );
}

void PrintList() {
    Node *cur = nil->next;
    while ( cur != nil ) {
        if ( cur != nil->next ) {
            printf( " " );
        }
        printf( "%d", cur->key );
        cur = cur->next;
    }
    printf( "\n" );
}

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 ( com[6] == 'F' ) {
                DeleteFirst();
            } else if ( com[6] == 'L' ) {
                DeleteLast();
            } else {
                DeleteKey( key );
                nd++;
            }
            size--;
        }
    }

    PrintList();
    
    return 0;
}