leetcode 206. 反转链表(Reverse Linked List)

题目描述：

反转一个单链表。

示例:

    输入: 1->2->3->4->5->NULL
    输出: 5->4->3->2->1->NULL

进阶:

你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

---

解法：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    // method 1:
    ListNode* reverseList1(ListNode* head, ListNode* & tail){
        if(!head){
            tail = NULL;
            return NULL;
        }else if(!head->next){
            tail = head;
            return head;
        }else{
            ListNode* _tail = NULL;
            ListNode* nxt = head->next;
            head->next = NULL;
            ListNode* _head = reverseList1(nxt, _tail);
            _tail->next = head;
            tail = head;    // update the tail node
            return _head;
        }
    }
    
    // method 2:
    ListNode* reverseList2(ListNode* head) {
        if(head){
            ListNode * cur = head, * nxt = head->next;
            head->next = NULL;
            while(nxt){
                cur = nxt;
                nxt = nxt->next;    // head, cur-->nxt
                cur->next = head;   // head<--cur, nxt
                head = cur;
            }
        }
        return head;
    }
    
    ListNode* reverseList(ListNode* head) {
        // ListNode* tail = NULL;
        // return reverseList1(head, tail);    // accepted
        return reverseList2(head);
    }
};