Problem p02265 (C++) — buggy submission, verdict: Runtime Error

#include <iostream>
#include <string>

using namespace std;

class Linked_list {
  struct Item {
    int num;
    Item *next;
    Item *prev;
  };
  Item *first, *last;

public:
  Linked_list() {
    first = NULL;
    last = NULL;
  }

  void insert(int x) {
    Item *item = new Item();
    item->num = x;
    item->prev = NULL;
    item->next = first;
    if (item->next != NULL)
      item->next->prev = item;
    first = item;
    if (last == NULL)
      last = item;
  }

  void delete_item(int x) {
    Item *item = first;
    while (item != NULL) {
      if (item->num == x) {
        if (item->prev != NULL) {
          item->prev->next = item->next;
        } else {
          first = item->next;
        }
        if (item->next != NULL) {
          item->next->prev = item->prev;
        } else {
          last = item->prev;
        }
        delete item;

        return;
      }
      item = item->next;
    }
  }

  void delete_first() {
    Item *item = first;
    if (first->next == NULL)
      last = NULL;
    first = item->next;
    delete item;
    if (first != NULL)
      first->prev = NULL;
  }

  void delete_last() {
    Item *item = last;
    if (last->prev == NULL)
      first = NULL;
    last = item->prev;
    if (last != NULL)
      last->next = NULL;
    delete item;
  }

  int pop_first() {
    Item *item = first;
    first = item->next;
    return item->num;
  }

  bool is_empty() { return first == NULL; }
};

int main() {
  ios::sync_with_stdio(false);
  Linked_list list;
  int n;
  string ope;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> ope;
    if (ope == "insert") {
      int x;
      cin >> x;
      list.insert(x);
    } else if (ope == "delete") {
      int x;
      cin >> x;
      list.delete_item(x);
    } else if (ope == "deleteFirst") {
      list.delete_first();
    } else if (ope == "deleteLast") {
      list.delete_last();
    }
  }

  int count = 0;
  while (!list.is_empty()) {
    if (count)
      cout << " ";
    cout << list.pop_first();
    count++;
  }
  cout << endl;
}

Error: 0

Fixed submission:

#include <iostream>
#include <string>

using namespace std;

class Linked_list {
  struct Item {
    int num;
    Item *next;
    Item *prev;
  };
  Item *first, *last;

public:
  Linked_list() {
    first = NULL;
    last = NULL;
  }

  void insert(int x) {
    Item *item = new Item();
    item->num = x;
    item->prev = NULL;
    item->next = first;
    if (item->next != NULL)
      item->next->prev = item;
    first = item;
    if (last == NULL)
      last = item;
  }

  void delete_item(int x) {
    Item *item = first;
    while (item != NULL) {
      if (item->num == x) {
        if (item->prev != NULL) {
          item->prev->next = item->next;
        } else {
          first = item->next;
        }
        if (item->next != NULL) {
          item->next->prev = item->prev;
        } else {
          last = item->prev;
        }
        delete item;

        return;
      }
      item = item->next;
    }
  }

  void delete_first() {
    Item *item = first;
    if (first->next == NULL)
      last = NULL;
    first = item->next;
    delete item;
    first->prev = NULL;
  }

  void delete_last() {
    Item *item = last;
    if (last->prev == NULL)
      first = NULL;
    last = item->prev;
    if (last != NULL)
      last->next = NULL;
    delete item;
  }

  int pop_first() {
    Item *item = first;
    first = item->next;
    return item->num;
  }

  bool is_empty() { return first == NULL; }
};

int main() {
  ios::sync_with_stdio(false);
  Linked_list list;
  int n;
  string ope;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> ope;
    if (ope == "insert") {
      int x;
      cin >> x;
      list.insert(x);
    } else if (ope == "delete") {
      int x;
      cin >> x;
      list.delete_item(x);
    } else if (ope == "deleteFirst") {
      list.delete_first();
    } else if (ope == "deleteLast") {
      list.delete_last();
    }
  }

  int count = 0;
  while (!list.is_empty()) {
    if (count)
      cout << " ";
    cout << list.pop_first();
    count++;
  }
  cout << endl;
}