合并两个有序链表

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
         ListNode node1(-1);
         ListNode* head = &node1;
    head->next = NULL;
    ListNode* p = head;

    if(NULL == l1 && NULL == l2)
    {
      return NULL;
    }

    if(NULL != l1 && NULL == l2)
    {
      return l1;
    }

    if(NULL == l1 && NULL != l2)
    {
      return l2;
    }

    while(NULL != l1 && NULL != l2)
    {

       if(l1->val > l2->val)
       {
           p->next = l2;
           l2 = l2->next;
           p = p-> next;
       }

       else
       {
           p->next = l1;
           l1 = l1->next;
           p = p-> next;
       }
    }

    if(NULL == l1)
    {
      p -> next = l2;
    }

    if(NULL == l2)
    {
      p -> next = l1;
    }


    p = head -> next;
   // free(head);

    return p;
        
        
    }