Find length of loop/cycle in given Linked List

// C# program to count number of nodes
// in loop in a linked list if loop is present
using System;
using System.Collections.Generic;

class Node {
    public int data;
    public Node next;
    public Node(int x) {
        data = x;
        next = null;
    }
}

class GfG {

    // This function detects and counts loop
    // nodes in the list. If loop is not there
    // then returns 0
    public static int countNodesinLoop(Node head) {

        HashSet<Node> visited = new HashSet<Node>();
        Node current = head;
        int count = 0;

        while (current != null) {

            // If the node is already visited, 
            // it means there is a loop
            if (visited.Contains(current)) {
                Node startOfLoop = current;
                do {
                    count++;
                    current = current.next;
                } while (current != startOfLoop);
                return count;
            }

            // Mark the current node as visited
            visited.Add(current);

            // Move to the next node
            current = current.next;
        }

        // Return 0 to indicate that
        //   there is no loop
        return 0;
    }

    public static void Main(string[] args) {

        Node head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.next.next.next = new Node(5);
        head.next.next.next.next.next = head.next;

        Console.WriteLine(countNodesinLoop(head));
    }
}

// C# program to count number of nodes
// in loop in a linked list if loop is present
using System;

class Node {
    public int data;
    public Node next;
    public Node(int x) {
        data = x;
        next = null;
    }
}

class GfG {

    // Returns count of nodes present in loop.
    public static int countNodes(Node node) {
        int res = 1;
        Node curr = node;
        while (curr.next != node) {
            res++;
            curr = curr.next;
        }
        return res;
    }

    // This function detects and counts loop
    //  nodes in the list. If loop is not there
    //  then returns 0 
    public static int countNodesinLoop(Node head) {
        Node slow = head, fast = head;

        while (slow != null && fast != null 
               && fast.next != null) {

            slow = slow.next;
            fast = fast.next.next;

            // If slow and fast meet at
            // some point then there is a loop
            if (slow == fast)
                return countNodes(slow);
        }

        // Return 0 to indicate that
        // there is no loop
        return 0;
    }

    public static void Main(string[] args) {
        
        Node head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.next.next.next = new Node(5);

        // Loop from 5 to 2
        head.next.next.next.next.next = head.next;
        Console.WriteLine(countNodesinLoop(head));
    }
}