Assigning value only works outside the loop

I am learning CPP (first language) and I am trying to reverse a linked list. this code is not working

node* reverse(node** head){
    node* previous=NULL;
    node* current=*head;
    node* nextptr=current->next;
    while(current!=NULL){
        current->next=previous;
        previous=current;
        current=nextptr;
        nextptr=nextptr->next;
    }
    return previous;
}

this one is working

node* reverse(node** head){
    node* previous=NULL;
    node* current=*head;
    node* nextptr;
    while(current!=NULL){
        nextptr=current->next;
        current->next=previous;
        previous=current;
        current=nextptr;
    }
    return previous;
}

Why does the second code snippet works while the first one doesn't ?

Why does the second code snippet works while the first one doesn't ?

The first snippet doesn't check before it dereferences potentially null pointers. Because you are using a null pointer to indicate the end of the list, it always dereferences a null pointer, and so has undefined behaviour.

The second snippet never dereferences a pointer it hasn't verified is non-null.