# Python 3 program to find the number
# of nodes in loop in a linked list
# if loop is present
 
# Python Code to detect a loop and
# find the length of the loop
# Node defining class
 
 
class Node:
 
    # Function to make a node
    def __init__(self, val):
        self.val = val
        self.next = None
 
# Linked List defining and loop
# length finding class
 
 
class LinkedList:
 
    # Function to initialize the
    # head of the linked list
    def __init__(self):
        self.head = None
 
    # Function to insert a new
    # node at the end
    def AddNode(self, val):
        if self.head is None:
            self.head = Node(val)
        else:
            curr = self.head
            while(curr.next):
                curr = curr.next
            curr.next = Node(val)
 
    # Function to create a loop in the
    # Linked List. This function creates
    # a loop by connecting the last node
    # to n^th node of the linked list,
    # (counting first node as 1)
    def CreateLoop(self, n):
 
        # LoopNode is the connecting node to
        # the last node of linked list
        LoopNode = self.head
        for _ in range(1, n):
            LoopNode = LoopNode.next
 
        # end is the last node of the Linked List
        end = self.head
        while(end.next):
            end = end.next
 
        # Creating the loop
        end.next = LoopNode
 
    # Function to detect the loop and return
    # the length of the loop if the returned
    # value is zero, that means that either
    # the linked list is empty or the linked
    # list doesn't have any loop
    def detectLoop(self):
 
        # if linked list is empty then there
        # is no loop, so return 0
        if self.head is None:
            return 0
 
        # Using Floyd’s Cycle-Finding
        # Algorithm/ Slow-Fast Pointer Method
        slow = self.head
        fast = self.head
        flag = 0  # to show that both slow and fast
        # are at start of the Linked List
        while(slow and slow.next and fast and
              fast.next and fast.next.next):
            if slow == fast and flag != 0:
 
                # Means loop is confirmed in the
                # Linked List. Now slow and fast
                # are both at the same node which
                # is part of the loop
                count = 1
                slow = slow.next
                while(slow != fast):
                    slow = slow.next
                    count += 1
                return count
 
            slow = slow.next
            fast = fast.next.next
            flag = 1
        return 0  # No loop
 
 
# Setting up the code
# Making a Linked List and adding the nodes
myLL = LinkedList()
myLL.AddNode(1)
myLL.AddNode(2)
myLL.AddNode(3)
myLL.AddNode(4)
myLL.AddNode(5)
 
# Creating a loop in the linked List
# Loop is created by connecting the
# last node of linked list to n^th node
# 1<= n <= len(LinkedList)
myLL.CreateLoop(2)
 
# Checking for Loop in the Linked List
# and printing the length of the loop
loopLength = myLL.detectLoop()
if myLL.head is None:
    print("Linked list is empty")
else:
    print(str(loopLength))
 
# This code is contributed by _Ashutosh

Time complexity: linear
Space complexity: constant