链表头 C++

Question

I am making a simple linked list for a data structures class and am having trouble understanding how the head pointer is supposed to work.

I have

template <typename E>
class SSLL{
template<typename E>
        struct Node {
            E data;
            Node* next;
        };
public:
template <typename E>
    SSLL();
    void push_front(E element);
private:
    Node<E> * head;
    Node<E> * tail;
};
template <typename E>
SSLL<E>::SSLL() {
    head = NULL;
    tail = NULL;
}
template <typename E>
void SSLL<E>::push_front(E element) {
    Node<E> * n = new Node<E>;
    n->data = element;
    n->next = head;
    head = n;
    if (!tail) {
        tail = n;
    }
}

However, doesn't this make head the first element in the list instead of a pointer to the first element.

I tried to do change push_front(E element) to this but am getting null pointer errors.

template <typename E>
void SSLL<E>::push_front(E element) {
    Node<E> * n = new Node<E>;
    n->data = element;
    n->next = head->next;
    head->next = n;
    if (!tail) {
        tail->next = n;
    }
}

All the examples I find online have head = n, but I still have trouble understanding why it is like that instead of head->next = n.

Thank you.

Answer 1

In most forms of linked lists, the head pointer points to the first node in the list or is null:

         +---+     +---+     +--+  
head --> | A | --> | B | --> |/0|  
         +---+     +---+     +--+  
                     ^  
                     |  
tail ----------------+  

Many beginners confuse a list with a node. A node is the object containing the data. A list is a collection of nodes.

Pushing at the head
Inserting at the front requires the following steps:

1. Making the new node point to the head node.
2. Changing the head node to point to the new node.

1) Making new node point to head node:

         +---+  
p_new -->| C |
         +---+  
           |  
           V  
         +---+     +---+     +--+  
head --> | A | --> | B | --> |/0|  
         +---+     +---+     +--+  

2) Making the head point to the new node:

         +---+  
p_new -->| C |  
head  -->|   |
         +---+  
           |  
           V  
         +---+     +---+     +--+  
         | A | --> | B | --> |/0|  
         +---+     +---+     +--+  

In C++, this would look like:

Node * p_node = new Node;
p_node->next = head; // Step 1.
head = p_node; // Step 2.

Answer 2

We can make new Head in c++ by simple code :

ListNode* dummy = new ListNode(0);
dummy->next = head;