Linked list c++ insertback - c++

Question

I am really new to data structures. I am trying to figure out why my insertback() function doesn't work. The first print does 3,2,1 but the second doesn't print anything. I think it has something to do with head, but I'm not really sure. Please help.
#include <iostream>
using namespace std;
struct Node
{
int data;
Node* next;
};
class lst
{
public:
void Insertfron(int x);
void Print();
void Insertback(int x);
private:
Node* head;
};
void lst::Insertfron(int x)
{
Node* temp = new Node;
temp->data = x;
temp->next = head;
head = temp;
}
void lst::Print()
{
Node* temp = head;
while(temp->next!=NULL)
{
cout<<temp->data<<' ';
temp=temp->next;
}
cout<< endl;
}
void lst::Insertback(int x)
{
Node* backinst = new Node;
backinst->data = x;
backinst->next = NULL;
Node* temp = head;
while(temp->next!=NULL)
{
temp = temp->next;
}
temp->next = backinst;
}
int main()
{
lst listt;
listt.Insertfron(1);
listt.Insertfron(2);
listt.Insertfron(3);
listt.Print();
listt.Insertback(4);
listt.Print();
return 0;
}

Answer 1

You are not initializing head to NULL to indicate an empty list, so ``head` will have a random garbage value, and thus all of your methods exhibit undefined behavior.
Once that is fixed, your while loops in both Print() and Insertback() are buggy, as they are not account for head being NULL when the list is empty.
Also, you are leaking every node you create. You need to add a destructor to free the nodes when you are done using the list.
With that said, try something more like this instead:
#include <iostream>
using namespace std;
struct Node
{
int data;
Node* next;
};
class lst
{
public:
lst();
~lst();
void Insertfron(int x);
void Print();
void Insertback(int x);
private:
Node* head;
};
lst::lst()
: head(NULL)
{
}
lst::~lst()
{
while (head != NULL)
{
Node *next = head->next;
delete head;
head = next;
}
}
void lst::Insertfron(int x)
{
Node* temp = new Node;
temp->data = x;
temp->next = head;
head = temp;
}
void lst::Print()
{
Node* temp = head;
while (temp != NULL)
{
cout << temp->data << ' ';
temp = temp->next;
}
cout << endl;
}
void lst::Insertback(int x)
{
Node* backinst = new Node;
backinst->data = x;
backinst->next = NULL;
if (head == NULL)
{
head = backinst;
}
else
{
Node* temp = head;
while (temp->next != NULL)
{
temp = temp->next;
}
temp->next = backinst;
}
}
int main()
{
lst listt;
listt.Insertfron(1);
listt.Insertfron(2);
listt.Insertfron(3);
listt.Print();
listt.Insertback(4);
listt.Print();
return 0;
}
That being said, Insertback() can be simplified to avoid the extra if by using an extra level of pointer indirection:
void lst::Insertback(int x)
{
Node **temp = &head;
while (*temp != NULL)
{
temp = &((*temp)->next);
}
Node* backinst = new Node;
backinst->data = x;
backinst->next = NULL;
*temp = backinst;
}