leetcode之链表-单链表反转-C++

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

迭代版本：

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.	class Solution {
10.	public:
11.	    ListNode* reverseList(ListNode* head) {
12.	        ListNode *p;
13.	        ListNode *q;
14.	        ListNode *r;
15.
16.	        if(head == NULL || head->next == NULL)
17.	        {
18.	            return head;
19.	        }
20.	        else
21.	        {
22.	            p = head;
23.	            q = head->next;
24.	            head->next = NULL;
25.
26.	            while(q)
27.	            {
28.	                r = q->next;
29.	                q->next = p;
30.	                p = q;
31.	                q = r;
32.	            }
33.	            head = p;
34.	        }
35.	        return head;
36.	    }
37.	};