LinkedList2.cpp

// https://practice.geeksforgeeks.org/problems/detect-loop-in-linked-list/1/
// Remove Linked List Loop
#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;
}

bool detectLoop(Node *head)
{
    Node *slow, *fast;
    slow = fast = head;
    bool flag = false;
    while (fast != NULL and slow != NULL)
    {
        slow = slow->next;
        fast = (fast->next);
        if (fast == NULL)
        {
            break;
        }
        fast = fast->next;
        if (fast == slow)
        {
            flag = true;
            break;
        }
    }
    return flag;
}