LeetCode Reverse Linked List

绝对老题了，感觉已经重复了

Reverse a singly linked list.

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Hint:

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

递归版本

 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  // 19:22

10 class Solution {

11 public:

12     ListNode* reverseList(ListNode* head) {

13         return reverseList(NULL, head);

14     }

15     

16     ListNode* reverseList(ListNode* last, ListNode* head) {

17         if (head == NULL) {

18             return last;

19         }

20         ListNode* rest = head->next;

21         ListNode* nhead = reverseList(head, rest);

22         head->next = last;

23         return nhead;

24     }

25 };

非递归版本

 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  // 19:20

10 class Solution {

11 public:

12     ListNode* reverseList(ListNode* head) {

13         if (head == NULL) {

14             return head;

15         }

16         ListNode* pre = NULL;

17         ListNode* cur = head;

18         while (cur != NULL) {

19             ListNode* tmp = cur->next;

20             cur->next = pre;

21             pre = cur;

22             cur = tmp;

23         }

24         return pre;

25     }

26 };