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>

struct node{
    int key;
    struct node *next;
    struct node *prev;
};
typedef struct node * nodePtr;

void init(void);
void insert(int key);
void delete(int target);
void deleteFirst(void);
void deleteLast(void);
void printList(void);

int main(void)
{
    init();

    int num_op;
    scanf("%d", &num_op);

    char op[12];
    int key;

    while (num_op--) {
        scanf("%s", op);
        if (strcmp(op, "insert") == 0) {
            scanf("%d", &key);
            insert(key);
        } else if (strcmp(op, "delete") == 0) {
            scanf("%d", &key);
            delete(key);
        } else if (strcmp(op, "deleteFirst") == 0) {
            deleteFirst();
        } else {
            deleteLast();
        }
    }

    printList();
    return 0;
}

    /*
       ------- root <------
      |                    |
       --> 3 --> 2 --> 1 --
    */

nodePtr root;
void init(void)
{
    root = malloc(sizeof(struct node));
    root->next = root;
    root->prev = root;
}

void insert(int key)
{
    nodePtr inserted;
    inserted = malloc(sizeof(struct node));
    inserted->next = root->next;
    inserted->prev = root;
    inserted->key = key;

    root->next->prev = inserted;
    root->next = inserted;
}

void deleteNode(nodePtr node);

void delete(int target)
{
    nodePtr current = root->next;
    while (current != root) {
        if (current->key == target) {
            deleteNode(current);
            return ;
        }
        current = current->next;
    }
}

void deleteNode(nodePtr node)
{
    if (node == root) {
        return ;
    }
    node->next->prev = node->prev;
    node->prev->next = node->next;
    free(node);
}

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

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

void printList(void)
{
    nodePtr current = root->next;
    int isFirst = 1;
    while (current != root) {
        if (isFirst == 0) {
            printf(" ");
        }
        printf("%d", current->key);
        current = current->next;
        isFirst = 0;
    }
    printf("\n");
}