detect_loop

// driver code

#include <bits/stdc++.h>
using namespace std;

struct Node
{
    int data;
    Node* next;
    
    Node(int val)
    {
        data = val;
        next = NULL;
    }
};

void loopHere(Node* head, Node* tail, int position)
{
    if(position==0) return;
    
    Node* walk = head;
    for(int i=1; i<position; i++)
        walk = walk->next;
    tail->next = walk;
}

bool detectLoop(Node* head);

int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int n, num;
        cin>>n;
        
        Node *head, *tail;
        cin>> num;
        head = tail = new Node(num);
        
        for(int i=0 ; i<n-1 ; i++)
        {
            cin>> num;
            tail->next = new Node(num);
            tail = tail->next;
        }
        
        int pos;
        cin>> pos;
        loopHere(head,tail,pos);
        
        if( detectLoop(head) )
            cout<< "True\n";
        else
            cout<< "False\n";
    }
	return 0;
}
// } Driver Code Ends



bool detectLoop(Node* head)
{
    Node *s=head,*f=head;
    while(s!=NULL && f!=NULL && f->next!=NULL)
    {
        s=s->next;
        f=f->next->next;
        if(s==f)
        return 1;
        
    }
    return 0;
}