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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