递归版

class Solution {
public:
    // 1.递归版
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode* ret = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;
        return ret;
    }
};

迭代版

class Solution {
public:
    // 2.迭代版
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode *pre = nullptr, *cur = head, *ne = head->next;
        while(ne){
            cur->next = pre;
            pre = cur;
            cur = ne;
            ne = ne->next;
        }
        cur->next = pre;
        return cur;
    }
};