输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

输入：head = [1,2]
输出：[2,1]

示例 3：

输入：head = []
输出：[]

 简单题 不用画图就出来了属于是 作业好多数据结构的文章还在创作中

struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode* prev = NULL;
    struct ListNode* cur = head;
    while(cur){
        struct ListNode* temp = cur->next;
        cur->next = prev;
        prev = cur;
        cur = temp;
    }
    return prev;
}

递归，好家伙这个想了好半天确实不好理解！

struct ListNode* reverseList(struct ListNode* head) {
    if(NULL == head || NULL == head->next){
        return head;
    }
    struct ListNode* ret = reverseList(head->next);
    head->next->next = head;
    head->next = NULL;
    return ret;
}