How to adjust head, tail pointers when building a LinkedList - c++

Question

I am making a simple implementation of LinkedList. My Trial:
#include<bits/stdc++.h>
using namespace std;
class ListNode
{
public:
ListNode* next;
int val;
ListNode(int x) : val(x), next(NULL) {}
};
int main()
{
ListNode* head = NULL;
ListNode* tail;
int data;
cout<<"Enter data. Enter -1 to terminate insertion"<<endl;
while(1)
{
cin>>data;
if(data != -1)
{
if(head == NULL)
{
head = new ListNode(data); // Returns the address of a new ListNode and stores it in head
tail = head->next; // tail now points to head's next
}
else
{
tail = new ListNode(data); // address of a new ListNode is in tail
tail = tail->next; // Tail now points to the new ListNode's next
}
}
else
break;
}
tail = NULL; // Tail ends with a NULL
while(head)
{
cout<<head->val<<" ";
head = head->next;
}
}
When I input 1, 2, 3: I expect the Linked List to be formed as 1->2->3->NULL.
However, the Linked List is always only the first element 1->NULL
I ran on the debugger and indeed, head->next is always NULL. But I dont understand why. I am specifically changing head's next to a new ListNode non-null address when I do tail = new ListNode(data), but apparently thats not happening. Where am I going wrong?
Here is the code: http://cpp.sh/6ardx

Answer 1

Problem: tail is always NULL.
How do you want to make the connection between tail and appended node to your list when tail is NULL ?
When list is empty and you create the first node, after inserting first node head and tail should point to the same node. Change
if(head == NULL)
{
head = new ListNode(data); // Returns the address of a new ListNode and stores it in head
tail = head->next; // tail now points to head's next
}
to
if(head == NULL)
{
tail = head = new ListNode(data); // Returns the address of a new ListNode and stores it in head
}
the second issue, when you add to the end of list you should update tail->next to point to the inserted node, so change
tail = new ListNode(data); // address of a new ListNode is in tail
tail = tail->next; // Tail now points to the new ListNode's next
to
tail->next = new ListNode(data); // address of a new ListNode is in tail
tail = tail->next; // Tail now points to the new ListNode's next