迭代版本

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)
            return head;
        auto a = head,b = a->next;
        while(b)
        {
            auto c = b->next;
            b->next = a;
            a = b, b = c;
        }
        head->next = NULL;
        return a;
    }
};

递归版本

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)
            return head;
        auto tail = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return tail;
    }
};