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



struct ListNode* reverseList(struct ListNode* head){
    struct ListNode *pre = NULL;
    struct ListNode *curr = head;
    while(curr)
    {
        struct ListNode *next = curr->next;
        curr->next = pre;
        pre = curr;
        curr = next;
    }
    return pre;
}