c++ - Reversing a Linked List with a void function

Question

I saw a approach to reverse a Linked list using recursion with a function but that function returns a node data type but I wanted to use a function with void return type and I made an approach -

void reverseLLrecursion(node* &head){
    if(head==NULL||head->next==NULL){        
        return ;
    }
    else{
        
        node* temp=head ;
        while(temp->next!=NULL){
             temp=temp->next ;
        }
        
        reverseLLrecursion(head->next) ;
        head->next->next=head ;
        head->next=NULL ;
        head=temp ;
    }

}

for a linked list 1->2->3->4->NULL it gives a output of 4->1->NULL. I don't understand where my code goes wrong. An explaination of the wrong output will be appreciated.

question from:https://stackoverflow.com/questions/65908257/reversing-a-linked-list-with-a-void-function

Answer 1

Basically you don't need this line:

        head=temp;

Notice, that "temp" always points to last node (called 4 by you), because you iterate up to next==NULL in while loop. There is no point in holding "temp" value.

When your recursion hits first condition:

head->next==NULL

it returns to the previous call, sets "next" pointer of the "next" node to current node (so '4' is pointing to '3'), and then sets current's node "next" pointer to NULL (this is important, because it hits '1' it has to point to NULL).

You can delete some of your code, this should do the trick:

    void reverseLLrecursion(node* &head){
        if(head->next==NULL){        
            return ;
        }
        else
        {
            reverseLLrecursion(head->next);
            head->next->next=head;
            head->next=NULL; 
        }
    }