链表//反转链表

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

递归：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null||head.next==null)
            return head;
        ListNode h = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return h;
    }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head==NULL||head->next==NULL)
            return head;
        ListNode *h = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return h;
    }
};

迭代：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode *p = head;
        ListNode *q = head->next;
        ListNode *r = head->next->next;
        p->next = NULL;
        while(r != NULL){
            q->next = p;
            p = q;
            q = r;
            r = r->next;
        }
        q->next = p;
        return q;
    }
};