find a loop in linked list

// C++ program to detect loop in a linked list 
#include <bits/stdc++.h> 
using namespace std; 
  
/* Link list node */
struct Node { 
    int data; 
    struct Node* next; 
}; 
  
void push(struct Node** head_ref, int new_data) 
{ 
    /* allocate node */
    struct Node* new_node = new Node; 
  
    /* put in the data  */
    new_node->data = new_data; 
  
    /* link the old list off the new node */
    new_node->next = (*head_ref); 
  
    /* move the head to point to the new node */
    (*head_ref) = new_node; 
} 
  
// Returns true if there is a loop in linked list 
// else returns false. 
bool detectLoop(struct Node* h) 
{ 
    unordered_set<Node*> s; 
    while (h != NULL) { 
        // If this node is already present 
        // in hashmap it means there is a cycle 
        // (Because you we encountering the 
        // node for the second time). 
        if (s.find(h) != s.end()) 
            return true; 
  
        // If we are seeing the node for 
        // the first time, insert it in hash 
        s.insert(h); 
  
        h = h->next; 
    } 
  
    return false; 
} 
  
/* Driver program to test above function*/
int main() 
{ 
    /* Start with the empty list */
    struct Node* head = NULL; 
  
    push(&head, 20); 
    push(&head, 4); 
    push(&head, 15); 
    push(&head, 10); 
  
    /* Create a loop for testing */
    head->next->next->next->next = head; 
  
    if (detectLoop(head)) 
        cout << "Loop found"; 
    else
        cout << "No Loop"; 
  
    return 0; 
} 
// This code is contributed by Geetanjali