leetCode:206 反转链表

206. 反转链表

题目：反转一个单链表。

进阶:
链表可以迭代或递归地反转。你能否两个都实现一遍？

非递归代码：

 class Solution {
     public ListNode reverseList(ListNode head) {
         if(head == null) return null;
         ListNode pre = null, nex = null;
         while(head != null){
             nex = head.next;
             head.next = pre;
             pre = head;
             head = nex;
         }
         return pre;
     }
 }

递归代码：

class Solution {
    public ListNode reverseList(ListNode head) {
        if(head==null||head.next ==null)
            return head;
        ListNode prev = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return prev;
    }
}