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>

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

Node *nil = NULL;

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

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

Node *searchNodeByKey(int key) {
    // printf(">> searchNodeByKey %d\n", key);
    Node *cur = nil->next;
    while (cur != nil) {
        if (cur->key == key)
            break;
        cur = cur->next;
    }
    return cur;
}

void deleteNode(Node *x) {
    // printf(">> deleteNode %d\n", x->key);
    if (x == nil)
        return;
    x->prev->next = x->next;
    x->next->prev = x->prev;
    free(x);
}

void deleteNodeByKey(int key) { deleteNode(searchNodeByKey(key)); }

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

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

void printList() {
    Node *cur = nil->next;
    while (1) {
        if (cur == nil)
            break;
        printf("%d", cur->key);
        if (cur->next != nil) {
            printf(" ");
        } else {
            printf("\n");
        }
        cur = cur->next;
    }
}

void deinit() {
    Node *cur = nil->next;
    while (cur != nil) {
        deleteNode(cur);
        cur = cur->next;
    }
    free(nil);
}

int main(int argc, char **argv) {
    int n = 0;
    char cmd[128] = {0};
    int key = 0;
    scanf("%d", &n);
    init();
    while (n > 0) {
        n--;
        scanf("%s %d", cmd, &key);
        // printf("cmd=%s key=%d\n", cmd, key);
        if (cmd[0] == 'i') {
            insertNodeByKey(key);
        } else if (cmd[6] == '\0') {
            deleteNodeByKey(key);
        } else if (cmd[6] == 'L') {
            deleteLast();
        } else {
            deleteFirst();
        }
        // printList();
    }
    printList();
    deinit();
    return 0;
}