LengthOfLoopInLinkedList.java

// 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 indeciate that ther is no loop*/
	return 0; 
} 

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

/* Driver program to test above function*/
	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.