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;
    struct node *next;
} Node;

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

// 双方向リストへの要素の挿入
void insert(int key) {
    Node *x = (Node *)malloc(sizeof(Node));
    x->key = key;

    // 番兵の直後に要素を追加
    x->next = nil->next;
    nil->next->prev = x;
    nil->next = x;
    x->prev = nil;
}

// 双方向リストの要素の探索
Node* listSearch(int key) {
    Node *cur = nil->next;   // 番兵の次の要素から辿る
    while (cur != nil && cur->key != key) {
        cur = cur->next;
    }
    return cur;
}

// 双方向リストの要素の削除
void deleteNode(Node *t) {
    if (t == nil) return;  // tが番兵の場合は処理しない
    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) {
    // 探索したノードを削除
    deleteNode(listSearch(key));
}

void printList() {
    Node *cur = nil->next;
    int count = 0;

    while(1) {
        if (cur == nil) break;
        if (count++ > 0) printf(" ");
        printf("%d", cur->key);
        cur = cur->next;
    }
    printf("\n");
 }

int main(void)
{

    int n, key, i;
    char command[100];

    init();

    scanf("%d", &n);

    i = 0;
    while(i < n) {
        scanf("%s %d", &command, &key);

        // 入力されたコマンドとキーでポインタを操作
        if (command[0] == 'i') {
            insert(key);
        } else if (command[0] == 'd') {
            if (command[6] == 'F') {
                deleteFirst();
            } else if (command[6] == 'L') {
                deleteLast();
            } else {
                deleteKey(key);
            }
        }
        i++;
    }

    printList();
 
    return 0;
}