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# 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
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def reverseList(self, head: ListNode) -> ListNode:
if head == None:
return head
i = head
if i.next == None:
return head
j = i.next
if j.next == None:
j.next = i
i.next = None
return j
k = j.next
while k.next != None:
j.next = i
i = j
j = k
k = k.next
j.next = i
k.next = j
head.next = None
return k