广州市从化区住房和建设局网站搜索引擎优化关键词选择的方法有哪些

题目描述：
定义一个函数，输入一个链表的头结点，反转该链表并输出反转后的链表的头结点。链表结点如下：

public class ListNode {int val;ListNode next = null;ListNode(int val) {this.val = val;}
}

分析：
要想反转链表，对于结点i，我们要把它的next指向它的前趋，因此我们需要保存前趋结点，同时，如果我们已经把i的next重新赋值，会无法找到i的后继，因此，在重新赋值之前，我们要保存i的后继。

代码如下：

/*** 反转链表* 题目：定义一个函数，输入一个链表的头节点，反转该链表并输入反转后该链表的头节点*/
public class ReverseList {public ListNode reverse(ListNode head){//如果输入的链表为空，就直接返回空if(head == null){return null;}//如果输入的链表只有一个节点，即：头节点。则直接返回该节点if(head.next == null){return head;}//定义反转后链表的头节点ListNode revHead = null;//当前节点ListNode curNode = head;while(curNode != null){ListNode tmpNode = curNode.next;        //记录下当前节点的下一个节点curNode.next = revHead;             //将当前节点的下一个节点反转指向反转链表的头节点revHead = curNode;curNode = tmpNode;}return revHead;}public static void main(String[] args) {ReverseList test = new ReverseList();ListNode head = new ListNode();ListNode temp1 = new ListNode();ListNode temp2 = new ListNode();ListNode temp3 = new ListNode();ListNode temp4 = new ListNode();ListNode temp5 = new ListNode();head.value = 1;temp1.value = 2;temp2.value = 3;temp3.value = 4;temp4.value = 5;temp5.value = 6;head.next = temp1;temp1.next = temp2;temp2.next = temp3;temp3.next = temp4;temp4.next = temp5;temp5.next = null;System.out.println("反转前：");test.printList(head);System.out.println();//执行反转操作ListNode reverseHead = test.reverse(head);test.printList(reverseHead);}//打印列表public void printList(ListNode head){while(head != null){System.out.print(head.value + "  ");head = head.next;}}
}class ListNode{int value;ListNode next;
}

使用递归实现：

/*** 使用递归实现：链表的反转*/
public class Solution {public static ListNode reverse(ListNode head) {// 参数校验if (head == null || head.next == null) {return head;}ListNode secondElem = head.next;head.next = null;ListNode revHead = reverse(secondElem);secondElem.next = head;return revHead;}public static void main(String[] args) {ListNode head = new ListNode();head.value = 1;ListNode node2 = new ListNode();node2.value = 2;ListNode node3 = new ListNode();node3.value = 3;ListNode node4 = new ListNode();node4.value = 4;ListNode node5 = new ListNode();node5.value = 5;ListNode node6 = new ListNode();node6.value = 6;head.next = node2;node2.next = node3;node3.next = node4;node4.next = node5;node5.next = node6;printList(head);System.out.println();ListNode revHead = reverse(head);printList(revHead);}// 打印列表public static void printList(ListNode head) {while (head != null) {System.out.print(head.value + "  ");head = head.next;}}}