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？

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

迭代：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        pre = None
        cur = head
        while(cur != None):
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre

递归：
 

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
	def reverseList(self, head):
		"""
		:type head: ListNode
		:rtype: ListNode
		"""
		if(head==None or head.next==None):
			return head
		cur = self.reverseList(head.next)
		head.next.next = head
		head.next = None
		return cur