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---
链表
---
#### 目录
1. 链表分类
- 单链表
- 循环链表
- 双向链表
- 双向循环链表
2. 常见链表算法题
- 单链表反转
- 链表中环的检测
- 两个有序链表的合并
- 删除链表倒数第 n 个结点
- 求链表的中间结点
3. 总结
#### 链表分类
##### 单链表
![](https://i.loli.net/2019/02/25/5c73a3d48b8f4.jpg)
##### 循环链表
![](https://i.loli.net/2019/02/25/5c73a4202fc70.jpg)
##### 双向链表
![](https://i.loli.net/2019/02/25/5c73a435a1dd8.jpg)
##### 双向循环链表
![](https://i.loli.net/2019/02/25/5c73a44c30c40.jpg)
#### 常见链表算法题
1. 单链表的反转
```java
public class LinkedListDemo {
private static Node head;
public static void main(String[] args) {
head = new Node(0);
head.next = new Node(1);
head.next.next = new Node(2);
head.next.next.next = new Node(3);
reverseLinkedList();
while (head != null) {
System.out.println(head.value);
head = head.next;
}
}
//单链表的反转
private static void reverseLinkedList() {
if (head == null || head.next == null) {
return;
}
Node p1 = head;
Node p2 = head.next;
Node p3 = null;
while (p2 != null) {
p3 = p2.next;
p2.next = p1;
p1 = p2;
p2 = p3;
}
head.next = null;
head = p1;
}
static class Node {
int value;
Node next;
Node(int value) {
this.value = value;
}
}
}
```
参考:[漫画:如何将一个链表“逆序”?](https://mp.weixin.qq.com/s/MR_qAbonFqGF_ljeWUC26w)
2. 链表中环的检测
这道题有三种解法:
```java
public class LinkedListDemo {
private static Node head;
public static void main(String[] args) {
head = new Node(0);
head.next = new Node(1);
head.next.next = new Node(2);
head.next.next.next = new Node(3);
head.next.next.next.next = head.next;
boolean isHasCycleBySet = hasCycleBySet();
System.out.println(isHasCycleBySet);
boolean isHasCycleByRun = hasCycleByRun();
System.out.println(isHasCycleByRun);
boolean isHasCycleByMagic = hasCycleByMagic();
System.out.println(isHasCycleByMagic);
}
//用 HashSet 判断
private static boolean hasCycleBySet() {
if (head == null || head.next == null) {
return false;
}
Set<Node> set = new HashSet<>();
while (head != null) {
if (set.contains(head)) {
return true;
} else {
set.add(head);
}
head = head.next;
}
return false;
}
//快慢指针
private static boolean hasCycleByRun() {
if (head == null || head.next == null) {
return false;
}
Node p1 = head;
Node p2 = head.next;
while (p1 != p2) {
if (p2 == null || p2.next == null) {
return false;
}
p1 = p1.next;
p2 = p2.next.next;
}
return true;
}
//魔法数解法
private static boolean hasCycleByMagic() {
if (head == null || head.next == null) {
return false;
}
while (head != null) {
if (head.value == 2 << 30) {
return true;
} else {
head.value = 2 << 30;
}
head = head.next;
}
return false;
}
static class Node {
int value;
Node next;
Node(int value) {
this.value = value;
}
}
}
```
3. 两个有序链表的合并
```java
```
4. 删除链表倒数第 n 个结点
```java
```
5. 求链表的中间结点
```java
```
#### 总结
面试中手写算法题最多的就是链表相关的,链表算法到底难不难?难?难得话一个题再写十遍还难不难?(气急败坏中...
不过,链表中需要注意几个边界条件:
1. 如果链表为空
2. 如果链表只有一个结点时
3. 如果链表只有两个结点时
4. 代码逻辑在处理头结点和尾结点时