大家看一下这个链表的类，为什么在构造函数中head = NULL，不是head -> next = NULL。如果用第二种的话会报错。
head只是一个指针，它为空表示它内部成员指针 next也为空吗？
class LinkedList {
public:
 struct Node {
   int val;
   Node* next;
   Node(int v, Node* n) { val = v; next = n; }
 };
private:
 Node* head;

public:
 LinkedList() {
     head = NULL;
 }
 ~LinkedList() {
     Node* q;
     for (Node* p = head; p != NULL; p=q) {
          q=p->next;
         delete[] p;
     }
 }

 void addFirst(int v) {
     Node* p = new Node(v, head);
     p->val = v;
     p->next = head;
     head = p;
 }
 void addLast(int v) {
   Node* p;
   Node* q;
   if (head == NULL) {
     head = new Node(v, NULL);
     return;
   }
   for (p = head; p != NULL; q=p, p = p->next){}
     
   q->next = new Node(v, NULL);
 }

 void remove(Node* p) {
     Node* q;
     for (q = head; q != NULL; q=q->next)
         if (q->next == p) {
             q->next = p->next;
             return;
     }
 }
 Node* getFirst() {
   return head;
 }
 Node* getLast() {
   Node* p;
   Node* q;
   for (p = head; p != NULL; q=p, p = p->next);
   return q;
 }
};