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

// -*- mode:c++; coding:utf-8; indent-tabs-mode:nil; -*-
// ALDS1_3-C: Doubly Linked List

#include <cassert>
#include <cstdio>

template <typename T> class doubly_linked_list {
  struct node {
    node *prev_;
    node *next_;
    T x_;
  };
  node sentinel_;

public:
  doubly_linked_list() { sentinel_.prev_ = sentinel_.next_ = &sentinel_; }
  void insert_elem_to_first(const T &x) {
    node *np = new node();
    np->x_ = x;
    np->prev_ = &sentinel_;
    np->next_ = sentinel_.next_;
    np->prev_->next_ = np;
    np->next_->prev_ = np;
  }
  void delete_first_elem(const T &x) { delete_node(search_first_elem(x)); }
  void delete_first_node() { delete_node(sentinel_.next_); }
  void delete_last_node() { delete_node(sentinel_.prev_); }

private:
  void delete_node(node *np) {
    if (np == &sentinel_) {
      return;
    }
    np->next_->prev_ = np->prev_;
    np->prev_->next_ = np->next_;
    delete np;
  }
  node *search_first_elem(const T &x) const {
    node *np = sentinel_.next_;
    for (; np != &sentinel_; np = np->next_) {
      if (np->x_ == x)
        break;
    }
    return np;
  }

public:
  void print(const char *sep = " ") const {
    const char *out_sep = "";
    for (const node *np = sentinel_.next_; np != &sentinel_; np = np->next_) {
      printf("%s%d", out_sep, np->x_);
      out_sep = sep;
    }
    puts("");
  }
};

int main(void) {
  unsigned int n;
  std::scanf("%d", &n);
  doubly_linked_list<int> list;

  for (auto i = 0U; i < n; ++i) {
    char buf[11 + 1]; // max_length == "deleteFirst".size()+1('\\0')
    int x;
    std::scanf("%11s", buf);
    if (buf[0] == 'i') { // insert x
      std::scanf("%d", &x);
      list.insert_elem_to_first(x);
    } else if (buf[6] == 'F') { // deleteFirst
      list.delete_first_node();
    } else if (buf[6] == 'L') { // deleteLast
      list.delete_last_node();
    } else { // (buf[6] == '\\0') // delete x
      std::scanf("%d", &x);
      list.delete_first_elem(x);
    }
  }
  list.print();

  return 0;
}
// end

Error: 0

Fixed submission:

// -*- mode:c++; coding:utf-8; indent-tabs-mode:nil; -*-
// ALDS1_3-C: Doubly Linked List

#include <cassert>
#include <cstdio>

template <typename T> class doubly_linked_list {
  struct node {
    node *prev_;
    node *next_;
    T x_;
  };
  node sentinel_;

public:
  doubly_linked_list() { sentinel_.prev_ = sentinel_.next_ = &sentinel_; }
  void insert_elem_to_first(const T &x) {
    node *np = new node();
    np->x_ = x;
    np->prev_ = &sentinel_;
    np->next_ = sentinel_.next_;
    np->prev_->next_ = np;
    np->next_->prev_ = np;
  }
  void delete_first_elem(const T &x) { delete_node(search_first_elem(x)); }
  void delete_first_node() { delete_node(sentinel_.next_); }
  void delete_last_node() { delete_node(sentinel_.prev_); }

private:
  void delete_node(node *np) {
    np->next_->prev_ = np->prev_;
    np->prev_->next_ = np->next_;
    delete np;
  }
  node *search_first_elem(const T &x) const {
    node *np = sentinel_.next_;
    for (; np != &sentinel_; np = np->next_) {
      if (np->x_ == x)
        break;
    }
    return np;
  }

public:
  void print(const char *sep = " ") const {
    const char *out_sep = "";
    for (const node *np = sentinel_.next_; np != &sentinel_; np = np->next_) {
      printf("%s%d", out_sep, np->x_);
      out_sep = sep;
    }
    puts("");
  }
};

int main(void) {
  unsigned int n;
  std::scanf("%d", &n);
  doubly_linked_list<int> list;

  for (auto i = 0U; i < n; ++i) {
    char buf[11 + 1]; // max_length == "deleteFirst".size()+1('\\0')
    int x;
    std::scanf("%11s", buf);
    if (buf[0] == 'i') { // insert x
      std::scanf("%d", &x);
      list.insert_elem_to_first(x);
    } else if (buf[6] == 'F') { // deleteFirst
      list.delete_first_node();
    } else if (buf[6] == 'L') { // deleteLast
      list.delete_last_node();
    } else { // (buf[6] == '\\0') // delete x
      std::scanf("%d", &x);
      list.delete_first_elem(x);
    }
  }
  list.print();

  return 0;
}
// end