len_loop_linked_list.cpp

#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 *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  
       their 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 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;  
}