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>

struct Node {
    int key;
    struct Node *next;
    struct Node *prev;
};
typedef struct Node node_t;
node_t *root;

int N;

void insert(int x)
{
    node_t *new = (node_t*)malloc(sizeof(node_t));
    new->key = x;
    new->next = root->next;
    new->prev = root;
    root->next->prev = new;
    root->next = new;
}

void deleteNode(node_t *p)
{
    p->prev->next = p->next;
    p->next->prev = p->prev;
    free(p);
}

void delete(int x)
{
    node_t *p = root->next;
    while (p != root) {
        if (p->key == x) {
            deleteNode(p);
            return;
        }
        p = p->next;
    }
}

void deleteFirst()
{
    deleteNode(root->next);
}

void deleteLast()
{
    deleteNode(root->prev);
}

void printList()
{
    node_t *p = root->next;
    
    // NULL ??§????????? root
    while (p != root) {
        printf("%d", p->key);
        p = p->next;
        if (p != root) {
            printf(" ");
        }
    }
    puts("");
}

int main()
{
    root = (node_t*)malloc(sizeof(node_t));
    root->next = root->prev = root;
    
    int i, j;
    char cmd[15];
    int x;
    
    scanf("%d", &N);
    for (i = 0; i < N; i++) {
        scanf("%s", cmd);
        switch (cmd[0]) {
        case 'i':
            scanf("%d", &x);
            insert(x);                
            break;
        case 'd':
            switch (cmd[6]) {
            case 'F':
                deleteFirst();
                break;
            case 'L':
                deleteLast();
                break;
            default:
                scanf("%d", &x);
                delete(x);
                break;
            }
            break;                
        }
    }
    
    printList();
    
    return 0;
}