迭代

/**
 * 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) {
        if (!head || !head->next) {
            return head;
        }

        auto a = head;
        auto b = a->next;
        while (b) {
            auto c = b->next;
            b->next = a;
            a = b;
            b = c;
        }
        head->next = nullptr;

        return a;
    }
};

递归

/**
 * 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) {
        if (!head || !head->next) {
            return head;
        }

        auto a = head->next;
        auto b = reverseList(a);
        a->next = head;
        head->next = nullptr;
        return b;
    }
};