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 <ctype.h>
#include <string.h>
 
typedef struct _node {
    int key;
    struct _node *next;
    struct _node *prev;
} NODE, *PNODE;
 
PNODE root;
 
void init(void) {
    root = malloc(sizeof(NODE));
    root->next = root;
    root->prev = root;
}
 
void insert(key) {
    PNODE t = malloc(sizeof(NODE));
    t->key = key;
    t->next = root->next;
    t->prev = root;
    root->next->prev = t;
    root->next = t;
}
 
void deleteNode(PNODE node) {
    if (node == root) {
        return;
    }
    node->next->prev = node->prev;
    node->prev->next = node->next;
    free(node);
}
 
void delete(key) {
    PNODE cur = root->next;
    while (cur != root) {
        if (cur->key == key) {
            deleteNode(cur);
            return;
        }
        cur = cur->next;
    }
}
void deleteFirst(void) {
    deleteNode(root->next);
}
 
void deleteLast(void) {
    deleteNode(root->prev);
}
 
void printNodes(void) {
    PNODE cur = root->next;
    while (1) {
        printf("%d", cur->key);
        cur = cur->next;
        if (cur != root) {
            putchar(' ');
        } else {
            break;
        }
    }
    putchar('\n');
}
 
void release(void) {
    PNODE tmp, cur = root->next;
    while (cur != root) {
        tmp = cur;
        cur = cur->next;
        deleteNode(tmp);        
    }
    free(root);
}
int main(void) {
    char cmd[100];
    int n, i, f;
    int key;
     
    init(); 
     
    scanf("%d", &n);
     
    f = 0;
    while (scanf("%s", cmd) > 0) {
        if (cmd[0] == 'i') {
            f = 1;
        } else if (cmd[0] == 'd') {
            switch (cmd[6]) {
            case '\0':
                f = 2;
                break;
            case 'F':
                deleteFirst();
                break;
            case 'L':
                deleteLast();
                break;
            }
        } else {
            key = atoi(cmd);
            if (f == 1) {
                insert(key);
            } else if (f == 2) {
                delete(key);
            }
        }
    }
     
    printNodes();
     
    release();
     
    return 0;
}