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？

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; */struct ListNode* reverseList(struct ListNode* head) {    if (NULL == head || NULL == head->next)    {        return head;    }    struct ListNode* a=head;    struct ListNode* b=head->next;    struct ListNode* c;        head->next = NULL;    while (b)    {        c = b->next;        b->next = a;        a = b;        b = c;    }    head = a;    return head;}