Finding loop in a singly linked-list

Question

How can I detect that whether a singly linked-list has loop or not?? If it has loop then how to find the point of origination of the loop i.e. the node from which the loop

Answer 1

Firstly, Create a Node

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

Initialize head pointer globally

Struct Node* head = NULL;

Insert some data in Linked List

void insert(int newdata){

    Node* newNode = new Node();
    newNode->data = newdata;
    newNode->next = head;
    head = newNode;
}

Create a function detectLoop()

void detectLoop(){
    if (head == NULL || head->next == NULL){
        cout<< "\nNo Lopp Found in Linked List";
    }
    else{
        Node* slow = head;
        Node* fast = head->next;
        while((fast && fast->next) && fast != NULL){
            if(fast == slow){
                cout<<"Loop Found";
                break;
            }
            fast = fast->next->next;
            slow = slow->next;
        }
        if(fast->next == NULL){
            cout<<"Not Found";
        }
    }
}

Call the function from main()

int main() 
{ 
    insert(4);
    insert(3);
    insert(2);
    insert(1);

    //Created a Loop for Testing, Comment the next line to check the unloop linkedlist
    head->next->next->next->next = head->next;

    detectLoop();
    //If you uncomment the display function and make a loop in linked list and then run the code you will find infinite loop 
    //display();
}