我无法使用以下代码反转给定的链表。 有人可以指出代码中的错误吗？

Question

I am unable to reverse the given linked list with the following code. Can someone point out the error in the code?

#include<iostream>
using namespace std;
class Node{
    public:
    int data;
    Node* next;
    Node* prev;
    Node(int d)
    {
        this->data=d;
        this->next=NULL;
        this->prev=NULL;
    }
};
void insertathead(Node* &head,int d)
{
    Node* temp= new Node(d);
    temp->next=head;
    head->prev=temp;
    head=temp;
}
void insertattail(Node* &tail, int d)
{
    Node* newnode=new Node(d);
    newnode->prev=tail;
    tail->next=newnode;
    tail=newnode;
}
void insertatmiddle(Node* &head, int position, int d)
{
    Node* temp=head;
    Node* newnode=new Node(d);
    int count=1;
    while(count<position-1)
    {
        temp=temp->next;
        count++;
    }
    newnode->prev=temp;
    temp->next->prev=newnode->next;
    newnode->next=temp->next;
    temp->next=newnode;
}
void reverse(Node* &head)
{
    Node* current=head;
    Node* temp=NULL;
    while(current!=NULL)
    {
       temp = current->prev; 
        current->prev = current->next; 
        current->next = temp;             
        current = current->prev;
    }
    head=temp->prev;
   

}
void display(Node* head)
{
    Node* temp=head;
    while(temp!=NULL)
    {
       cout<<temp->data<<" ";
       temp=temp->next;
    }
    cout<<endl;
}
int main()
{
  Node* node1=new Node(10);
  Node* head=node1;
  Node* tail=node1;
  insertathead(head,11);
  display(head);
  insertathead(head,12);
  display(head);
  insertathead(head,13);
  display(head);
  insertathead(head,14);
  display(head);
  insertattail(tail,15);
  display(head);
  insertatmiddle(head,3,20);
  display(head);
  cout<<head->data<<endl;
  cout<<tail->data<<endl;
  reverse(head);
  display(head);
  

  return 0;
}

In this code, when I am commenting out the insertatmiddle function then the reverse function is able to reverse the linked list completely. But after adding a node in between, the reverse function reverses the linked list till that particular node only. Can someone figure out the error in this case?

Answer 1

In your code for insertatmiddle

newnode->prev=temp;
temp->next->prev=newnode->next;  // newnode->next is always NULL here
newnode->next=temp->next;
temp->next=newnode;

you've made a mistake. Try

newnode->prev=temp;
newnode->next=temp->next;
temp->next->prev=newnode;
temp->next=newnode;

And by the way, if the code is for you, as long as you understand what you're doing, all good, but the way you implemented

int count=1;
while(count<position-1) { ... }

baffles me a bit, since the link temp which has the newnode appended after itself is head for position = 0 , 1 , and 2 , which I find very unusual.