链接列表插入

Question

I'm struggling with this example of linked lists, I am not able to change any of the code in the second code block below, however I can freely edit this function. Due to this I believe the use of nodes will not work as I cannot add them anywhere. Any ideas?

// Insert the given string into the linked-list such that the
// entries in the linked-list are in alphabetical order
bool List::insert(const char *string_p)
{
    // Please write the list insert function
  return SUCCESS;
}

Code that cannot be modified is below

class ListEntry
{
  public:
    explicit     ListEntry();
    explicit     ListEntry(const char *string_p);
                 ~ListEntry();
    string       getData();
    void         setData(const char* string_p);
    void         setData(string string);
    ListEntry   *getNext();
    ListEntry   *getPrevious();
    ListEntry   *prev_p;   // pointer to previous entry in the linked-list
    ListEntry   *next_p;   // pointer to next entry in the linked-list

  private:
    string          data;      // entry's string
};

// Represents the linked-list object
class List
{
  public:
    List();
    ~List();

    bool printForward();
    bool printReverse();
    bool insert(const char *string_p);

  private:
    int        entryCount;  // number of entries present in the linked-list
    ListEntry *head_p;      // pointer to the first entry in the list
    ListEntry *tail_p;      // pointer to the last entry in the list
};

// ListEntry constructor
ListEntry::ListEntry()
{
  this->prev_p = NULL;
  this->next_p = NULL;
  return;
}

// ListEntry constructor
ListEntry::ListEntry(const char *string_p)
{
  this->data   = string_p;
  this->prev_p = NULL;
  this->next_p = NULL;
  return;
}

// List entry destructor 
ListEntry::~ListEntry()
{
  return;
}

// Return the stored string object
string ListEntry::getData()
{
  return this->data;
}

// Set the internal string data from a char*
void ListEntry::setData(const char* string_p)
{
  this->data = string_p;
}

// Set the internal string data from a string
void ListEntry::setData(string string)
{
  this->data = string;
}

// Returns reference to the next entry in the list
ListEntry *ListEntry::getNext()
{
  return this->next_p;
}

// Returns reference to the previous entry in the list
ListEntry *ListEntry::getPrevious()
{
  return this->prev_p;
}

// List constructor
List::List()
{
  this->entryCount = 0;
  this->head_p     = NULL;
  this->tail_p     = NULL;
}

// List destructor
List::~List()
{
  // Delete all entries in the list
  ListEntry *entry_p   = this->head_p;
  ListEntry *current_p = this->head_p;

  while (entry_p != NULL)
  {
    current_p = entry_p; 
    entry_p = entry_p->getNext();
    delete current_p;
  }
 }

// Output linked list in order from head to tail
// printing out the string data from each list entry
bool List::printForward()
{
  ListEntry *entry_p = this->head_p;
  int count = 0;

  cout << "FORWARD: " << this->entryCount << " entries\n";
  while (entry_p != NULL)
  {
    cout << entry_p->getData() << " ";

    if (++count % 5 == 0 || entry_p == this->tail_p)
    {
      cout << endl;
    }

    entry_p = entry_p->getNext();
  }

  return SUCCESS;
}

// Output linked list in reverse order from tail to head
// printing out the string data from each list entry
bool List::printReverse()
{
  ListEntry *entry_p = this->tail_p;
  int count = 0;

  cout << "REVERSE: " << this->entryCount << " entries\n";
  while (entry_p != NULL)
  {
    cout << entry_p->getData() << " ";

    if (++count % 5 == 0 || entry_p == this->head_p)
    {
      cout << endl;
    }

    entry_p = entry_p->getPrevious();
  }

  return SUCCESS;
}

Answer 1

So you have most of the hard stuff out of the way. What you have to do is complete the insert function witch has 2 special cases.

1) When head_p is null (list is empty) 2) When head_p is not null (list is not empty)

Using this in the insert method you take the const char *string_p you are given and create a ListEntry from it. From there you insert the created ListEntry into the list.

If head_p is null then you are basically creating the list and will need to set head and end pointers to the new ListEntry. If the list isn't empty you have to add it to the end. This requires the update of a the prev_p and next_p pointers in a ListEntry (leaving this as an exercise for you).