原链表不断取头，新链表头插

class Solution {
public:
    ListNode* ReverseList(ListNode* pHead) {
        ListNode* newHead = nullptr;
        while(pHead){
            //原链表取头
            ListNode* tmp = pHead;
            pHead = pHead->next;
            //新链表头插
            tmp->next = newHead;
            newHead = tmp;
        }
        return newHead;
    }
};