C++ Program For Finding The Length Of Loop In Linked List

// 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 *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 indicate that 
       there 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; 
} 
// This code is contributed by SHUBHAMSINGH10