Java Program For Finding The Length Of Loop In Linked List

// Java program to count number of nodes 
// in loop in a linked list if loop is 
// present 
import java.io.*;

class GFG 
{
    // Link list node
    static class Node 
    { 
        int data;
        Node next;
        Node(int data)
        {
            this.data =data;
            next =null;
        }
    }

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

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

        while (slow_p !=null && 
               fast_p!=null && 
               fast_p.next!=null) 
        { 
            slow_p = slow_p.next; 
            fast_p = fast_p.next.next; 

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

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

    static Node newNode(int key) 
    { 
        Node temp = new Node(key);    
        return temp; 
    } 

    // Driver code
    public static void main (String[] args) 
    {
        Node head = newNode(1); 
        head.next = newNode(2); 
        head.next.next = newNode(3); 
        head.next.next.next = newNode(4); 
        head.next.next.next.next = newNode(5); 

        // Create a loop for testing 
        head.next.next.next.next.next = head.next; 

        System.out.println( countNodesinLoop(head)); 
    }
}
// This code is contributed by inder_verma.