Inserting inside a Linked List - c++

Question

My insertAfter function in the LinkedList is not working right.
I would really appreciate if someone could point out what's wrong with my code.
Thanks :)
Here is the list struct:
struct ListItem
{
T value;
ListItem<T> *next;
ListItem<T> *prev;
ListItem(T theVal)
{
this->value = theVal;
this->next = NULL;
this->prev = NULL;
}
};
and here is my function to insert inside the list:
template <class T>
void LinkedList<T>::insertAfter(T toInsert, T afterWhat)
{
if(head!=NULL)
{
ListItem<T>*p;
p=head;
while(p->next!=NULL)
{
if(p->value==afterWhat)
{
ListItem<T>*k;
k=new ListItem<T>(toInsert);
k->next=p->next;
p->next=k;
k->prev=p;
k->next->prev=k;
}
else
p=p->next;
}
}
else
{
head= new ListItem<T>(toInsert);
}
//p=NULL;
}

Answer 1

Firstly, no insertion should be made if, afterWhat is not found.
So, there will be no insertion when either: the list is empty, or afterWhat does not exist in the list.
Secondly, while(p->next!=NULL) loop will never check the last element of the list for afterWhat, because it ends its iterations one before the last.
So, if afterWhat is the last element in the list, it will go undetected.
Thirdly, the loop should be broken as soon as the insertion is done. Later the value of p can also determine whether afterWhat was found or not, and appropriate result can be displayed, if need be.
Fourth, now since the case of afterWhat being the last element is also being handled, we also need to check p->next!=NULL before making any changes to it, because it p could be the last element, in which case, nothing needs to be done for p->next.
Note that, you cannot insert an element in the head position using the insertAfter function, because nothing is before head, which can be used as afterWhat in this case.
The insertAfter function, should go like this:
template <class T>
void LinkedList<T>::insertAfter(T toInsert, T afterWhat)
{
if(head != NULL)
{
ListItem<T>*p;
p=head;
while(p != NULL)
{
if(p->value==afterWhat)
{
ListItem<T>*k;
k = new ListItem<T>(toInsert);
k->next = p->next;
p->next = k;
k->prev=p;
if(k->next != NULL)
k->next->prev = k;
break;
}
else
p = p->next;
}
if(p == NULL)
cout<<"afterWhat, not found, hence no insertion made\n");
else
cout<<"Inserted!\n";
}
else
{
cout<<"List is empty, hence afterWhat, not found\n");
}
}