the value does not change recursion during the stack unwinding

Question

void reverseLinkedList(Node* &head) 
{
    if (head == nullptr || head->next == nullptr) 
    {
        return ;
    }

    Node* Rest = head->next;

    reverseLinkedList(Rest);
    head->next->next = head; 
    head->next = nullptr;   
    head = Rest;
}



int main() 
{
    Node* h = nullptr;

    Create(h, 4);//create linked-list with N nodes
    reverseLinkedList(h);
    Print(h);
}

according to the debugging, the Rest variable still has the same value during the stack unwinding. does it is supposed to be changed every call stack? but it still has the same value in each call.

Answer 1

the Rest variable still has the same value during the stack unwinding.

This is expected.

When the last recursive call occurs, the local variable Rest is initialised to the tail node, which after that call returns is assigned to the pass-by-reference head variable. This head reference is the Rest variable of the caller, so also that caller's local Rest now points to the tail. And so the unwinding continues where each head = Rest copies that tail reference to the caller's Rest variable (because head is a pass-by-reference parameter).

That head = Rest assignment really is like the "portal" back to the caller. And so each caller gets their own Rest variable to point to the original tail node, which is now the head of the reversed list.