图解链表倒置

链表倒置是一个经常遇到的问题，下面将用图示的方式讲述链表倒置的过程。

假设链表的长度为4，头指针head指向第一个节点。另外还有两个非常重要的指针，prev：带领head指针往下走，是个领航员，worker：位于head之后，为指针变换服务。最后返回的结果是一个head指针，指向的是一个已经倒置的链表。

1. 初始化：

Node *prev = null;
Node *worker = null;

2. 开始工作

prev = head.next;
worker = head;

 

 

3.  准备第一次指针变换

head = prev;
prev = head.next;

4. 变换指针

head.next = worker;
worker.next = null;

 

5. 准备第二次变换

worker = head;
head = prev;
prev = head.next;

 

6. 第二次指针变换

head.next = worker;

 

7. 准备第三次变换

worker = head;
head = prev;

 

8. 第三次变换

head.next = worker;

 

至此，已经变换完成。在一步步的代码中发现了一丝重复了吗？

让我们把之前每一步的代码都合在一起：

Node *prev = null;
Node *worker = null;
prev = head.next;
worker = head;
head = prev;
prev = head.next;
head.next = worker;
worker.next = null;
worker = head;
head = prev;
prev = head.next;
head.next = worker;
worker = head;
head = prev;
head.next = worker;

试着在第三行，第八行，第十二行之后，加一个空行。像这样：

Node *prev = null;
Node *worker = null;
prev = head.next;

worker = head;
head = prev;
prev = head.next;
head.next = worker;
worker.next = null;

worker = head;
head = prev;
prev = head.next;
head.next = worker;

worker = head;
head = prev;
head.next = worker;

发现一丝重复的迹象了吗？现在我们将初始化部分，也即第八行删去，因为这个指针不影响我们的结果，我们在初始化就将它赋值为空，也即在第三行后面加上一句，head.next = null; 之后，将最后一次的过程也改一下，在倒数第二行加上一句 prev = head.next;因为此时head已经指向最后一个节点，head.next = null, 也即 prev = null;

改造之后的代码为：

Node *prev = null;
Node *worker = null;
prev = head.next;
head.next = null;

worker = head;
head = prev;
prev = head.next;
head.next = worker;


worker = head;
head = prev;
prev = head.next;
head.next = worker;

worker = head;
head = prev;
prev = head.next;
head.next = worker;

嗯，现在非常清晰了。在加上判断条件吧，就以最后一次的操作为截至，此时，三个指针的值为：

prev = null
head = 最后一个节点
worker = 倒数第二个节点

那么以 head.next != null 为判断条件就可以了。

让我们来组装一下代码吧：

Node* ReverseList(Node *head) {
        Node *prev = null;
        Node *worker = null;
         
        if(head == null) {
            return null;
        }
 
        if(head.next == null) {
            return head;
        }
 
        prev = head.next;
        head.next = null;
        while(prev != null) {
            worker = head;
            head = prev;
            prev = head.next;
            head.next = worker;
        }
 
        return head;
         
 }

大功告成！