LeetCode 第206题 反转链表

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？

 1 class Solution206 {
 2 
 3   //迭代
 4   public ListNode reverseList(ListNode head) {
 5     ListNode dummyHead = new ListNode(0);
 6     dummyHead.next = head;
 7     ListNode node = head;
 8     ListNode newDummyHead = new ListNode(0);
 9 
10     while (node != null) {
11       dummyHead.next = node.next;
12       node.next = newDummyHead.next;
13       newDummyHead.next = node;
14       node = dummyHead.next;
15     }
16     return newDummyHead.next;
17   }
18 
19   //递归
20   public ListNode reverseList_2(ListNode head) {
21     if (head == null || head.next == null) {
22       return head;
23     }
24     ListNode newHead = reverseList_2(head.next);
25     head.next.next = head;
26     head.next = null;
27     return newHead;
28 
29   }
30 
31 }