迭代版

/**
 * 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) {
        // 检测是否为空
        if(!head || !head->next) return head;

        auto p = head, q = p->next;
        while(q){
            auto o = q->next;
            q->next = p;
            p=q,q=o;
        }

        head->next = NULL;
        return p;
    }
};

递归版

/**
 * 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) {
        // 检测是否为空
        if(!head || !head->next) return head;
        if(head==NULL || head->next==NULL) return head;

        ListNode* res = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return res;
    }
};