class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    def print(self):
        tmp = self.head
        while tmp:
            print(tmp.data, end=", ")
            tmp = tmp.next
        print()

llist = LinkedList()
llist.head = Node(1)

second = Node(2)
third = Node(3)
llist.head.next = second
second.next = third

llist.print() # 1, 2, 3,

# in 1->2->3->4->5->6 find 4 
slow = fast = head
while fast and fast.next:
    slow = slow.next
    fast = fast.next.next

prev, curr = None, head
while curr:
    tmp = curr.next
    curr.next = prev
    prev = curr
    curr = tmp