Reverse Linked List****

Reverse a singly linked list.

Hint:

A linked list can be reversed either iteratively or recursively. Could you implement both?

分析：p = p->next的位置很重要！！继续思考！

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* reverseList(ListNode* head) {
12         ListNode* p = head;
13         ListNode* current = NULL;
14         while(p!=NULL)
15         {
16             ListNode* newNode = p;
17             p = p->next;
18             newNode->next = current;
19             current = newNode;
20         }
21         return current;
22     }
23 };

Runtime: 8ms

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* reverseList(ListNode* head) {
12         if(!head || !head->next) return head;
13         
14         ListNode* pre = head;
15         ListNode* current = pre->next;
16         pre->next = NULL;
17         while(current){
18             ListNode* post = current->next;
19             current->next = pre;
20             pre = current;
21             current = post;
22         }
23         return pre;
24     }
25 };