反转链表

题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list

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？

迭代解法：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null)
            return null;
        ListNode pre = null;
        ListNode cur = head;
        ListNode next = cur.next;
        while(next != null){
            ListNode tmp = next.next;
            next.next = cur;
            cur.next = pre;
            pre = cur;
            cur = next;
            next = tmp;
        }
        return cur;
    }
}

递归解法：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null)
            return null;
        ListNode ans = new ListNode(0);
        reverse(head.next,head,ans);
        head.next = null;
        return ans.next;
    }

    private void reverse(ListNode cur,ListNode pre,ListNode ans){
        if(cur == null){
            ans.next = pre;
            return;
        }
        ListNode tmp = cur.next;
        cur.next = pre;
        reverse(tmp,cur,ans);
    }
}