lengthOfLoop.cpp

// C++ program to count number of nodes 
// in loop in a linked list if loop is 
// present 
#include<bits/stdc++.h> 
using namespace std; 

/* 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 *head) 
{ 
	 struct Node* fast= head;// minimum 2 node
    
    struct Node* slow = head; // minimum 3 node
    //end when fast and slow are equal
    bool loopExists=false;
    while(fast->next && fast->next->next){
        fast=fast->next->next;    
        slow=slow->next;
        if(fast==slow){
            loopExists=true;
            break;
        }
    }
    if(fast==slow && loopExists){
        Node* temp=fast;
        int count=0;
        while(temp->next!=fast){
            temp=temp->next;
            count++;
        }
        return count+1;
    }
    else{
        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 Code 
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; 

	cout << countNodesinLoop(head) << endl; 

	return 0; 
} 

// This code is contributed by SHUBHAMSINGH10