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>
#include <stdbool.h>

typedef struct _Node Node;
struct _Node {
        int val;
        Node *prev;
        Node *next;
};

Node *dummy;

Node *newNode(int _val, Node *_prev,  Node *_next) {
        Node *m = (Node*)malloc(sizeof(Node));
        m->val = _val;
        m->prev = _prev;
        m->next = _next;
        return m;
}

void init() {
        dummy = newNode(-1, NULL, NULL);
        dummy->prev = dummy;
        dummy->next = dummy;
}

Node* list_first() {
        return dummy->next;
}

Node* list_last() {
        return dummy->prev;
}

Node* list_end() {
        return dummy;
}

Node* insertAfter(Node *n, int val) {
        Node *m = newNode(val, n, n->next);
        n->next->prev = m;
        n->next = m;
        return m;
}

Node* InsertFirst(int val) {
        return insertAfter(dummy, val);
}

void Erase(Node *n) {
        if (n == dummy) {
                return;
        }
        n->prev->next = n->next;
        n->next->prev = n->prev;
        free(n);
        return ;
}

void Delete(int val) {
        Node *n = list_first();
        for (; n != list_end(); n = n->next) {
                if (n->val == val) {
                        return Erase(n);
                }
        }
}

void DeleteFirst() { Erase(list_first()); }

void DeleteLast() { Erase(list_last()); }

void Show() {
        Node *n = list_first();
        for (; n != list_end(); n = n->next) {
                printf(" %d", n->val);
        }
        printf("\n");
}

void PrintList() {
        bool ifs = false;
        Node *n;
        for (n = list_first(); n != list_end(); n = n->next) {
                if (ifs == true) {
                        printf(" ");
                }
                printf("%d", n->val);
                ifs = true;
        }
        printf("\n");
}

int main() {
        int loopmax, val, i;
        char cmd[64] = {};

        init();
        scanf("%d", &loopmax);
        for (i = 0; i < loopmax; i++) {
                memset(cmd, '\0', 64);
                scanf("%s %d", cmd, &val);
                if (cmd[0] == 'i') {
                        InsertFirst(val);
                } else {
                        if (cmd[6] == 'F') {
                                DeleteFirst();
                        }
                        else if (cmd[6] == 'L') {
                                DeleteLast();
                        }
                        else {
                                Delete(val);
                        }
                }
        }
        PrintList();

        return 0;
}