loop_remove.cpp

// C++ program to detect and remove loop 
#include <bits/stdc++.h> 
using namespace std; 

struct Node { 
	int key; 
	struct Node* next; 
}; 

Node* newNode(int key) 
{ 
	Node* temp = new Node; 
	temp->key = key; 
	temp->next = NULL; 
	return temp; 
} 

// A utility function to print a linked list 
void printList(Node* head) 
{ 
	while (head != NULL) { 
		cout << head->key << " "; 
		head = head->next; 
	} 
	cout << endl; 
} 

// Function to detect and remove loop 
// in a linked list that may contain loop 
void detectAndRemoveLoop(Node* head) 
{ 
	// If list is empty or has only one node 
	// without loop 
	if (head == NULL || head->next == NULL) 
		return; 

	Node *slow = head, *fast = head; 

	// Move slow and fast 1 and 2 steps 
	// ahead respectively. 
	slow = slow->next; 
	fast = fast->next->next; 

	// Search for loop using slow and 
	// fast pointers 
	while (fast && fast->next) { 
		if (slow == fast) 
			break; 
		slow = slow->next; 
		fast = fast->next->next; 
	} 

	/* If loop exists */
	if (slow == fast) { 
		slow = head; 
		while (slow->next != fast->next) { 
			slow = slow->next; 
			fast = fast->next; 
		} 

		/* since fast->next is the looping point */
		fast->next = NULL; /* remove loop */
	} 
} 

/* Driver program to test above function*/
int main() 
{ 
	Node* head = newNode(50); 
	head->next = head; 
	head->next = newNode(20); 
	head->next->next = newNode(15); 
	head->next->next->next = newNode(4); 
	head->next->next->next->next = newNode(10); 

	/* Create a loop for testing */
	head->next->next->next->next->next = head->next->next; 

	detectAndRemoveLoop(head); 

	printf("Linked List after removing loop \n"); 
	printList(head); 

	return 0; 
}