/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    
    ListNode* reverseList(ListNode* head) {
        if(head == nullptr || head->next == nullptr)
            return head;
        
        ListNode* pre = nullptr;
        ListNode* next = nullptr;
        
        while(head != nullptr && head->next!=nullptr)
        {
            // 记录head->next
            next = head->next;
            // head指向prev
            head->next = pre;
            // prev 向后移动到head
            pre = head;
            // head指向next
            head = next;
        }
        head->next = pre;
        return head;
    }
};