题解 | #翻转链表#

import sys


class ListNode:
    def __init__(self,val=0,next=None):
        self.val=val
        self.next=next


def parse_input(slist):
    if not slist:
        return None
    head=ListNode(slist[0])
    curr=head
    for value in slist[1:]:
        curr.next=ListNode(value)
        curr=curr.next
    return head

def reverseList(head):
    if not head or not head.next:
        return head

    slow,fast=head,head
    while fast and fast.next:
        slow=slow.next
        fast=fast.next.next

    prev,curr=None,slow.next
    slow.next=None
    while curr:
        next_tmp=curr.next
        curr.next=prev
        prev=curr
        curr=next_tmp
    
    first,second=head,prev
    while second:
        tmp1,tmp2=first.next,second.next
        first.next=second
        second.next=tmp1
        first,second=tmp1,tmp2
    return head

s=list(map(int,input().strip().split(',')))
# print(s)
head=parse_input(s)
head=reverseList(head)
res=[]
while head:
    res.append(str(head.val))
    head=head.next
print(",".join(res))