java单链表常用操作

总结提高，与君共勉

概述、

数据结构与算法亘古不变的主题，链表也是面试常考的问题，特别是手写代码常常出现，将从以下方面做个小结

【链表个数】

【反转链表-循环】

【反转链表-递归】

【查找链表倒数第K个节点】

【查找链表中间节点】

【判断链表是否有环】

【从尾到头打印单链表-递归】

【从尾到头打印单链表-栈】

【由小到大合并有序单链表-循环】

【由小到大合并有序单链表-递归】

通常在java中这样定义单链表结构

<span style="font-family:Microsoft YaHei;font-size:14px;">class Node{int value;Node next;public Node(int n){this.value = n;this.next = null;}}</span>

1、链表个数

这个比较简单，不再赘述

<span style="font-family:Microsoft YaHei;font-size:14px;">// 求单链表的结点个数public int getListLen(Node head) {int len = 0;while (head != null) {len++;head = head.next;}return len;}</span>

2、反转链表-循环

采用双指针，主要是4行代码，其中2,3俩行完成指针反转，1,4主要是保持head往下指

<span style="font-family:Microsoft YaHei;font-size:14px;">// 反转单链表(循环)public Node reverseList(Node head) {// 安全性检查if (head == null || head.next == null)return head;Node pre = null;Node temp = null;while (head != null) {// 以下1234均指以下四行代码temp = head.next;// 与第4行对应完成头结点移动head.next = pre;// 与第3行对应完成反转pre = head;// 与第2行对应完成反转head = temp;// 与第1行对应完成头结点移动}return pre;}</span>

3、反转链表-递归

<span style="font-family:Microsoft YaHei;font-size:14px;">// 将单链表反转,递归public static Node reverseListRec(Node head) {if (head == null || head.next == null)return head;Node reHead = reverseListRec(head.next);head.next.next = head;head.next = null;return reHead;}</span>

4、查找链表倒数第K个节点

双指针法，不多解释

<span style="font-family:Microsoft YaHei;font-size:14px;">// 查找单链表倒数第K个结点 双指针法public Node reKNode(Node head, int k) {if (head == null)return head;int len = getListLen(head);if (k > len)return null;Node targetK = head;Node nextK = head;// 先走到K个位置for (int i = 0; i < k; i++) {nextK = nextK.next;}// 再和头结点一起走，nextk走到结尾，此时targetk为倒数第K个节点while (nextK != null) {nextK = nextK.next;targetK = targetK.next;}return targetK;}</span>

5、查找链表中间节点

快慢指针，不多解释

<span style="font-family:Microsoft YaHei;font-size:14px;">public Node getMid(Node head) {// 类似的快慢指针法// 安全性检查if (head == null || head.next == null)return head;Node target = head;Node temp = head;while (temp != null && temp.next != null) {target = target.next;temp = temp.next.next;}return target;}</span>

6、判断链表是否有环

主要还是快慢指针，如果快的指针能够追上慢指针则有环

<span style="font-family:Microsoft YaHei;font-size:14px;">// 判断一个单链表中是否有环,快慢指针public boolean hasCycle(Node head) {boolean flag = false;Node p1 = head;Node p2 = head;while (p1 != null && p2 != null) {p1 = p1.next;p2 = p2.next.next;if (p2 == p1) {flag = true;break;}}return flag;}</span>

7、从尾到头打印单链表-递归

<span style="font-family:Microsoft YaHei;font-size:14px;">// 从尾到头打印单链表(递归)public void reList1(Node head) {// 安全性检查if (head == null)return;else {reList1(head.next);System.out.println(head.value);}}</span>

8、从尾到头打印单链表-栈

利用栈FILO的性质，先存储节点然后输出每个栈的节点值

<span style="font-family:Microsoft YaHei;font-size:14px;">// 从尾到头打印单链表（栈）public void reList2(Node head) {Stack<Node> s = new Stack<Node>();while (head != null) {s.push(head);head = head.next;}while (!s.isEmpty()) {System.out.println(s.pop().value);}}</span>

9、由小到大合并有序单链表-循环

<span style="font-family:Microsoft YaHei;font-size:14px;">// 由小到大合并俩个有序的单链表(循环)public Node mergeSort1(Node head1, Node head2) {// 安全性检查if (head1 == null)return head2;if (head2 == null)return head1;// 新建合并节点Node target = null;// 确定第一个元素的节点if (head1.value > head2.value) {target = head2;head2 = head2.next;} else {target = head1;head1 = head1.next;}target.next = null;// 开始合并Node mergeHead = target;while (head1 != null && head2 != null) {// 当两个链表都不为空if (head1.value > head2.value) {target.next = head2;head2 = head2.next;} else {target.next = head1;head1 = head1.next;}target = target.next;target.next = null;}if (head1 == null)target.next = head2;elsetarget.next = head1;return mergeHead;}</span>

10、由小到大合并有序单链表-递归

<span style="font-family:Microsoft YaHei;font-size:14px;">// 由小到大合并俩个有序的单链表(递归)public Node mergeSort2(Node head1, Node head2) {if (head1 == null)return head2;if (head2 == null)return head1;if (head1.value > head2.value) {head2.next = mergeSort2(head2.next, head1);return head2;} else {head1.next = mergeSort2(head1.next, head2);return head1;}}</span>