solution.cpp

#include <bits/stdc++.h>

using namespace std;

class SinglyLinkedListNode {
    public:
        int data;
        SinglyLinkedListNode *next;

        SinglyLinkedListNode(int node_data) {
            this->data = node_data;
            this->next = nullptr;
        }
};

class SinglyLinkedList {
    public:
        SinglyLinkedListNode *head;
        SinglyLinkedListNode *tail;

        SinglyLinkedList() {
            this->head = nullptr;
            this->tail = nullptr;
        }

        void insert_node(int node_data) {
            SinglyLinkedListNode* node = new SinglyLinkedListNode(node_data);

            if (!this->head) {
                this->head = node;
            } else {
                this->tail->next = node;
            }

            this->tail = node;
        }
};

void print_singly_linked_list(SinglyLinkedListNode* node, string sep, ofstream& fout) {
    while (node) {
        fout << node->data;

        node = node->next;

        if (node) {
            fout << sep;
        }
    }
}

void free_singly_linked_list(SinglyLinkedListNode* node) {
    while (node) {
        SinglyLinkedListNode* temp = node;
        node = node->next;

        free(temp);
    }
}

/*The function below detects cycles in a singly linked list.
  we have to write the below function in the problem*/

bool has_cycle(SinglyLinkedListNode* head) {
  SinglyLinkedListNode* fast = head;    //iterator to traverse through the linked list.
  SinglyLinkedListNode* slow = head;   //Another iterator but it will go behind fast iterator.
  while(slow != nullptr && fast != nullptr && fast->next!= nullptr)
  {
    slow = slow->next ;  //slow is going through each node.
    fast = fast->next->next; //fast is traversing through skipping each alternate node.Thus ahead of slow.
    if(slow==fast)
      {
        return true;  //if in any step slow and fast is pointing at the same node then cycle exists in the linked list.Hence return true. 
      }
}
  return false;  //if loop runs it's life then there is no cycle as fast is either pointing at last element or nullptr.

}

int main()
{
    ofstream fout(getenv("OUTPUT_PATH"));

    int tests;
    cin >> tests;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');

    for (int tests_itr = 0; tests_itr < tests; tests_itr++) {
        int index;
        cin >> index;
        cin.ignore(numeric_limits<streamsize>::max(), '\n');

        SinglyLinkedList* llist = new SinglyLinkedList();

        int llist_count;
        cin >> llist_count;
        cin.ignore(numeric_limits<streamsize>::max(), '\n');

        for (int i = 0; i < llist_count; i++) {
            int llist_item;
            cin >> llist_item;
            cin.ignore(numeric_limits<streamsize>::max(), '\n');

            llist->insert_node(llist_item);
        }
      
      	SinglyLinkedListNode* extra = new SinglyLinkedListNode(-1);
      	SinglyLinkedListNode* temp = llist->head;
      
      	for (int i = 0; i < llist_count; i++) {
            if (i == index) {
          		extra = temp;
            }
          	
          	if (i != llist_count-1) {
          		temp = temp->next;
            }
        }
      
      	temp->next = extra;

        bool result = has_cycle(llist->head);

        fout << result << "\n";
    }

    fout.close();

    return 0;
}