165. 合并两个排序链表双指针

ListNode * mergeTwoLists(ListNode * l1, ListNode * l2) {
        if(!l1)
            return l2;
        if(!l2)
            return l1;
        
        
        ListNode *newhead=new ListNode(0);
        ListNode *newhead_back;
        ListNode *head1=l1;
        ListNode *head2=l2;
        ListNode *tmp1;
        ListNode *tmp2;
        
        while((head1!=NULL)&&(head2!=NULL))   //两个都不空，才进while循环
        {
            tmp1=head1;
            tmp2=head2;
            
            //tmp1->next=NULL;
            //tmp2->next=NULL;
            
            if(tmp1->val<tmp2->val)
            {
                
                head1=head1->next;
                tmp1->next=NULL;
                if(!newhead->next)
                {
                    newhead->next=tmp1;
                    newhead_back=newhead->next;
                }
                else
                {
                newhead_back->next=tmp1;
                newhead_back=newhead_back->next;
                }
                
            }
            else 
            {
                
                head2=head2->next;
                tmp2->next=NULL;
                if(!newhead->next)
                {
                    newhead->next=tmp2;
                    newhead_back=newhead->next;
                }
                else
                {
                newhead_back->next=tmp2;
                newhead_back=newhead_back->next;
                }
                
            }
        }
        
        if(head1)     //如果head1不空，把其连接到后面
        {
            newhead_back->next=head1;
            return newhead->next;
        }
        if(head2)     //如果head2不空，把其连接到后面
        {
            newhead_back->next=head2;
            return newhead->next;
        }
        
        return newhead->next;
        // write your code here
    }