Reverse Linked List

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
1、迭代法，超时

 ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)return head;
        ListNode dummy(-1);
        ListNode* prev = &dummy;
        prev->next = head;
        ListNode* cur = head->next;   
        while(cur->next)
        {
            ListNode* tmp = cur;
            cur->next = tmp->next;
            
            tmp->next = dummy.next;
            dummy.next = tmp;
            
        }
        return dummy.next;
    }

 2、递归(https://www.cnblogs.com/grandyang/p/4478820.html)

  ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)return head;
        
        ListNode* p = head;
        head = reverseList(head->next);
        p->next->next = p;
        p->next = nullptr;
        return head;
    }