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

class Solution(object):
    def reverseList(self, head):
        """
        迭代
        """
        res = None
        while head:
            next_node = head.next
            head.next, res = res, head
            head = next_node
        return res

    def reverseList2(self, head):
        """递归"""
        def _run(head, res):
            if not head:
                return res
            next_node = head.next
            head.next, res = res, head
            return _run(next_node, res)
        return _run(head, None)
    
