关于哨兵节点的介绍

为了简化处理链表边界条件而引入的附加链表结点&＃xff0c;哨兵节点通常位于链表头部&＃xff0c;它的值没有任何意义&＃xff0c;在一个有哨兵节点的链表中&＃xff0c;从第二个节点开始才真正保存有意义的信息。
具体作用看代码和注释&＃xff1a;

public class ListNode {public int val;public ListNode next;public ListNode(int val) {this.val &＃61; val;}&＃64;Overridepublic String toString() {return "ListNode{" &＃43;"val&＃61;" &＃43; val &＃43;", next&＃61;" &＃43; next &＃43;&＃39;}&＃39;;}
}

public class Sentry {// 哨兵节点可以简化创建或删除链表头节点操作的代码public ListNode append1(ListNode head,int value){ListNode newNode &＃61; new ListNode(value);if (head&＃61;&＃61;null){return newNode;}ListNode node &＃61; head;while (node.next !&＃61;null){node &＃61; node.next;}node.next &＃61; newNode;return head;}
// 由于新创建的一个哨兵节点当作链表的头结点&＃xff0c;链表无论如何也不会为空&＃xff0c;因此不需要使用if语句单独处理
// 输入头节点head为null的情况&＃xff0c;哨兵节点简化了代码的逻辑。public ListNode append2(ListNode head,int value){ListNode dummy &＃61; new ListNode(0);dummy.next &＃61; head;ListNode newNode&＃61; new ListNode(value);ListNode node &＃61; dummy;while (node.next !&＃61;null){node &＃61; node.next;}node.next &＃61; newNode;return dummy.next;}public ListNode delete1(ListNode head,int value){if (head &＃61;&＃61; null){return head;}if (head.val &＃61;&＃61; value){return head.next;}ListNode node &＃61; head;while (node.next !&＃61; null){if (node.next.val &＃61;&＃61; value){node.next &＃61; node.next.next;break;}node &＃61; node.next;}return head;}public ListNode delete2(ListNode head,int value){ListNode dummpy &＃61; new ListNode(0);dummpy.next &＃61; head;ListNode node &＃61; dummpy;while (node.next !&＃61; null){if (node.next.val &＃61;&＃61; value){node.next &＃61; node.next.next;break;}node &＃61; node.next;}return dummpy.next;}}