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 Cell {
    int key;
    struct Cell* prev;
    struct Cell* next;
} Cell;

Cell* newCell (int key) {
    Cell *cell = malloc(sizeof(Cell));
    cell->key = key;
    cell->prev = NULL;
    cell->next = NULL;
    return cell;
}

Cell* tail, *head, *deleted;

void init() {
    tail = head = deleted = NULL;
}

void insert(int x) {
    if (tail == NULL) {
      tail = head = newCell(x);
    } else {
      tail->prev = newCell(x);
      tail->prev->next = tail;
      tail = tail->prev;
    }
}

void deleteLast() {
   if (head == NULL) return;
   deleted = head;
   if (head->prev == NULL) {
       tail = head = NULL;
       free(deleted);
       return;
   }
   
   head = head->prev;
   head->next = NULL;
   free(deleted);
}

void deleteFirst() {
    if (tail == NULL) return;
    deleted = tail;
    
    if (tail->next == NULL) {
        tail = head = NULL;
        free(deleted);
        return;
    }
    
    tail->next->prev = NULL;
    tail = tail->next;
}


void dump() {
    Cell* tmp = tail;
    int n = 0;
    while (tmp != NULL) {
        if (n!=0) printf(" ");
        printf("%d", tmp->key);
        tmp = tmp->next;
        n++;
    }
    printf("\n");
}

void delete(int x) {
    Cell* tmp = tail;
  
    while (tmp != NULL) {
        if (tmp->key != x) {
            tmp = tmp->next;
            continue;
        }
        

        if (tmp == tail) {
            deleteFirst();
            break;
        } else if (tmp == head) {
            deleteLast();
            break;
        }
        
        deleted = tmp;
        Cell* prev = tmp->prev;
        Cell* next = tmp->next;
        prev->next = next;
        next->prev = prev;
        free(deleted);
        break;
    }
}


void test() {
    init();
    insert(5); // 5
    insert(2); // 2 5
    insert(3); // 3 2 5
    insert(1); // 1 3 2 5
    delete(3); // 1 2 5
    insert(6); // 6 1 2 5
    delete(5); // 1 2 5
    deleteFirst(); // 1 2 
    deleteLast(); // 1
    dump();
}

int main(void){
    // test();return 0;
    int n, x;
    char op[15];
    
    init();
    
    scanf("%d", &n);
    while (n--) {
      scanf("%s", &op);

      if (strcmp(op, "insert") == 0) {
          scanf("%d", &x);
          insert(x);
      } else if (strcmp(op, "delete") == 0) {
          scanf("%d", &x);
          delete(x);
      } else if (strcmp(op, "deleteLast") == 0) {
          deleteLast();
      } else {
          deleteFirst();
      }
    }
    
    dump();
}