No.206 Reverse Linked List

 1 #include "stdafx.h"
 2 
 3 struct ListNode
 4 {
 5     int val;
 6     ListNode *next;
 7     ListNode(int x):val(x),next(NULL){}
 8 };
 9 class Solution
10 {
11 public:
12     ListNode* reverseList(ListNode* head)
13     {//单链表反转
14      //带头结点的单链表：head里面存放链表长度（或其他信息），head->next指向第一个实际节点；
15         if(!(head) || !(head->next))//如果head为空，或者头结点指向空节点（链表长度为0）
16             return head;
17 
18         ListNode *current = head->next;
19         ListNode *back = NULL;//逆转之后，头结点变为尾节点，其next为Null  
20         ListNode *front;
21         //current 记录当前位置，back记录上一个位置，为current->next的值；front记录下一个位置，反转后current不等于current->next  
22         while(!current)
23         {
24             front = current->next;//反转后不能再用current->next，所以先记录下这个节点  
25             current->next = back;
26             back = current;
27             current = front;
28         }
29         head->next = back; //current已经为空，所以back为尾节点。head->next指向它。
30         return head;
31     }
32 };

 1 class Solution
 2 {
 3 public:
 4     ListNode* reverseList(ListNode* head)
 5     {//单链表反转
 6      //不带头结点的单链表
 7         if(!(head) || !(head->next))
 8             return head;
 9 
10 //改：    ListNode *current = head->next;
11         ListNode *current = head;
12         ListNode *back = NULL;
13         ListNode *front;
14 
15         while(!current)
16         {
17             front = current->next;
18             current->next = back;
19             back = current;
20             current = front;
21         }
22 //改：    head->next = back;
23         head = back;
24         return head;
25     }
26 };