/**
 * 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) {
        ListNode *prev = nullptr;  // 定义空节点
        ListNode *cur = head;
        while(cur){
            ListNode *t = cur->next;
            cur->next = prev;
            prev = cur;
            cur = t;
        }
        return prev;
    }
};