ListNode* node = new ListNode(0,head);
the first member val to 0, and the second member next to head
ListNode* node = new ListNode(0);
new member 'node' val to 0
ListNode* node=new ListNode();
new member 'node' without initial val
在对链表进行操作时,一种常用的技巧是添加一个哑节点(dummy node),它的next 指针指向链表的头节点。这样一来,我们就不需要对头节点进行特殊的判断了。