206. 反转链表

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (head == nullptr) return nullptr;
        ListNode* a = nullptr;
        while (head != nullptr) {
            ListNode* t = head->next;
            head->next = a;
            a = head;
            head = t;
        }
        return a;
    }
};