Find length of loop in linked list

Write a function detectAndCountLoop() that checks whether a given Linked List contains loop and if loop is present then returns count of nodes in loop. For example, loop is present in below linked list and length of loop is 4. If loop is not present, then function should return 0.

Recommended: Please try your approach on PRACTICE, before moving on to the solution.

We know that Floyd’s Cycle detection algorithm terminates when fast and slow pointers meet at a common point. We also know that this common point is one of the loop nodes (2 or 3 or 4 or 5 in the above diagram). We store the address of this common point in a pointer variable say ptr. Then we initialize a counter with 1 and start from the common point and keeps on visiting next node and increasing the counter till we again reach the common point(ptr). At that point, the value of the counter will be equal to the length of the loop.

C

// C program to count number of nodes 
// in loop in a linked list if loop is 
// present 
#include<stdio.h> 
#include<stdlib.h> 
  
/* Link list node */
struct Node 
{ 
    int data; 
    struct Node* next; 
}; 
  
// Returns count of nodes present in loop. 
int countNodes(struct Node *n) 
{ 
   int res = 1; 
   struct 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 */
int countNodesinLoop(struct Node *list) 
{ 
    struct Node  *slow_p = list, *fast_p = list; 
  
    while (slow_p && fast_p && fast_p->next) 
    { 
        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; 
} 
  
struct Node *newNode(int key) 
{ 
    struct Node *temp = 
        (struct Node*)malloc(sizeof(struct Node)); 
    temp->data = key; 
    temp->next = NULL; 
    return temp; 
} 
  
/* Driver program to test above function*/
int main() 
{ 
    struct 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; 
  
    printf("%d \n", countNodesinLoop(head)); 
  
    return 0; 
} 

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. 

Python3

# Python 3 program to find the number 
# of nodes in looop 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 connnecting 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 

C#

// C# program to count number of nodes  
// in loop in a linked list if loop is  
// present  
using System; 
  
class GFG  
{ 
  
    /* Link list node */
    class Node  
    {  
        public int data; 
        public Node next; 
        public 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 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;  
  
        Console.WriteLine( countNodesinLoop(head));  
    } 
} 
  
// This code is contributed by Rajput-Ji 

Output :

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