链表中的add_sorted函数

Question

I am currently trying to make a linked list with a function that can add integers sorted to the list while using some of the functions I made, but I am running into a problem with my code:

void list::add_sorted(int el)
    {
      if (empty()) add_first(el);
      else if (el<=head->data) add_first(el);
      else if (el >= tail->data) add_last(el);
    }

And I am wondering why the output in my code: 3

While in the main function i wrote :

list t1;
t1.add_sorted(5);
t1.add_sorted(3);
t1.add_sorted(9);
t1.print();

Here is the implementation for add_first(int el) , add_last(int el) and empty():

list::list()
{
  head = tail = 0;

}
bool list::empty()
{
  return (head == 0);

}
void list::add_first(int el)
{
    if (empty()) head = tail = new node(el);
    else
     head = new node(el);

}
void list::add_last(int el)
{
    if (empty()) head = tail = new node(el);
    else
     tail=tail->next = new node(el);

}

Thanks in advance.

Answer 1

My understanding of an insertion into a sorted linked list is to keep the list sorted.

This means that you will need to search the list to find an appropriate location to add the new element:

void addSorted(int value)
{
  node * new_element = new node(el);
  new_element->next = nullptr;
  if (head == nullptr)
  {
    head = new_element;
    tail = head;
  }
  else
  {
     node * p = head;
     node * previous = head;
     while (p != nullptr)
     {
       if (el > p.value)
       {
          new_element->next = p;
          previous->next = new_element;
          break;
       }
     }
     if (p == nullptr)
     {
        previous.next = new_element;
     }
  }
}

The idea here is that you need to search the list for the insertion point, then add your new element. There are corner cases, such as when the head is null.

Answer 2

You have done a small mistake in your void list::add_first(int el) method. As you did not add you all codes I have modified your code and which is available below. Please compare the below codes to understand you mistake, especially compare method void list::add_first(int el) . Hope this will help.

struct node { public: int data; node *next; node(int val):data(val),next(NULL){} };

    class list
    {
      private:
         node *head,*tail, *temp;
      public:
         list();
         bool empty();
         void add_sorted(int el);
         void add_first(int el);
         void add_last(int el);
         void print();
    };

    list::list()
    {
      head = tail = 0;

    }
    void list::add_sorted(int el)
    {
       if (empty()) add_first(el);
       else if (el<=head->data) add_first(el);
       else if (el >= tail->data) add_last(el);
    }
    bool list::empty()
    {
      return (head == 0);

    }
    void list::add_first(int el)
    {
        if (empty()) head = tail = new node(el);
        else
        {
         //here temp is pointing to the first node of the list
         temp = head;
         head = new node(el); //now head is pointing to a new node
         head->next = temp; // and that new node is added to the list and finally head is pointing to the first node of the final list
        }

    }
    void list::add_last(int el)
    {
        if (empty()) head = tail = new node(el);//here I dont think this condition will ever satisfied
        else
         tail->next = new node(el);
         tail = tail->next;//more clearity

    }
    void list::print()
    {
      temp = head;
      while(temp)
      {
        std::cout<<temp->data<<", ";
        temp = temp->next;
      }
      std::cout<<std::endl;
    }
    int main()
    {
      list t1;
      t1.add_sorted(5);
      t1.add_sorted(3);
      t1.add_sorted(9);
      t1.print();
      return 0;
    }