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 }