I am getting an segmentation fault error in line head=NULL - c++

Question

I didn't understand why i am getting error in line head=NULL I am just pointing head towards NULL
I have also tried creating a new node for head and then pointing towards NULL but still it doesn't work
Code show here
#include <bits/stdc++.h>
using namespace std;
struct node
{
int data;
node *next;
};
node *head=NULL;
node *tail=NULL;
void createnode(int value)
{
node *temp=new node;
temp->data=value;
temp->next=NULL;
if(head==NULL)
{
head=temp;
tail=temp;
temp=NULL;
}
else
{
tail->next=temp;
tail=temp;
}
}
void deletek(int n,int k)
{
if(head==NULL)
return;
node *temp;
temp=head;
while(temp->data<temp->next->data)
{
head=temp->next;
temp=temp->next;
k--;
}
while(temp!=NULL&&k!=0)
{
if(temp->next->data<temp->next->next->data)
{
temp->next=temp->next->next;
k--;
}
temp=temp->next;
}
}
void display()
{
node *temp=head;
while(temp!=NULL)
{
cout<<temp->data<<" ";
temp=temp->next;
}
cout<<endl;
}
int main() {
int t;
cin>>t;
int n,k;
for(int i=0;i<t;i++)
{
cin>>n>>k;
int arr[n];
for(int j=0;j<n;j++)
{
cin>>arr[j];
createnode(arr[j]);
}
deletek(n,k);
display();
head=NULL;
}
}
I expect that whenever the line head=NULL runs the previous linked list should get delete and I should be able to enter elements in the linked list from the beginning

Answer 1

First of all: C++ likes "nullptr" (since C++11) for this, instead of "NULL".
=> e.g. https://en.cppreference.com/w/cpp/types/NULL vs. https://en.cppreference.com/w/cpp/language/nullptr
Second: The exact compiler output could help.
Third: The list will not be deleted if you throw away the address of it's location. maybe you should rather create a class and/or add destructor for your node. If you want to implement LL by yourself
Or have a look at std::list -> https://en.cppreference.com/w/cpp/container/list. I use this rather often.
Or if you like to add boost dependency: https://www.boost.org/doc/libs/1_35_0/doc/html/intrusive/list.html more fancy but you'll add an external library
Then: The Segfault (on my PC) happens in if(temp->next->datanext->next->data) I guess you can never know if temp->next->next->data exists... maybe not even if temp->next->data => if temp->next points to NULL (or nullptr) it is not a valid instance of node to dereference
try:
`
#include <bits/stdc++.h>
#define HERE() printf("HERE: %d\n", __LINE__)
using namespace std;
struct node
{
int data;
node *next;
};
node *head=NULL;
node *tail=NULL;
void createnode(int value)
{
node *temp=new node;
temp->data=value;
temp->next=NULL;
if(head==NULL)
{
head=temp;
tail=temp;
temp=NULL;
}
else
{
tail->next=temp;
tail=temp;
}
}
void deletek(int n,int k)
{
if(head==NULL)
return;
node *temp;
temp=head;
while(temp->data<temp->next->data)
{
head=temp->next;
temp=temp->next;
k--;
}
HERE();
while(temp!=NULL&&k!=0)
{
HERE();
int d1 = temp->next->data;
HERE();
int d2 = temp->next->next->data;
if(temp->next->data < temp->next->next->data)
{
temp->next=temp->next->next;
k--;
}
temp=temp->next;
}
}
void display()
{
node *temp=head;
while(temp!=NULL)
{
cout<<temp->data<<" ";
temp=temp->next;
}
cout<<endl;
}
int main() {
int t;
cout << "input t:" << endl;
cin>>t;
int n,k;
for(int i=0;i<t;i++)
{
cout << "input n/k" << endl;
cin>>n>>k;
int arr[n];
for(int j=0;j<n;j++)
{
cout << "input arr[" << j <<"]" << endl;
cin>>arr[j];
createnode(arr[j]);
}
deletek(n,k);
display();
head=NULL;
}
}
`