Detect Loop

Hard Singly Linked List: Advanced PRO

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Problem Statement

Write a C program to detect if a loop exists in a singly linked list using Floyd’s Cycle-Finding Algorithm (print 1 if loop exists, 0 otherwise).

Input Format

First line contains n. Second line contains n elements. Third line contains loop position index (-1 if no loop, or 0-indexed position where last node connects).

Output Format

Print 1 if loop is detected, else 0.

Constraints

1 <= n <= 50

Sample Input

5
1 2 3 4 5
1

Sample Output

Explanation

Use slow and fast pointers. If they meet, a cycle exists; if fast reaches NULL, no cycle exists.

Starter Code

#include <stdio.h>
#include <stdlib.h>

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

int main() {
    int n;
    if (scanf("%d", &n) == 1) {
        struct Node *head = NULL, *tail = NULL;
        struct Node *loopNode = NULL;
        for (int i = 0; i < n; i++) {
            int val;
            scanf("%d", &val);
            struct Node *newNode = (struct Node *)malloc(sizeof(struct Node));
            newNode->data = val;
            newNode->next = NULL;
            if (head == NULL) {
                head = newNode;
                tail = newNode;
            } else {
                tail->next = newNode;
                tail = newNode;
            }
            if (i == 1) loopNode = newNode; // example tracking for test hooks
        }
        int loopPos;
        scanf("%d", &loopPos);
        // Write your code here to construct loop if loopPos >= 0 and detect loop
    }
    return 0;
}

Limits

Time Limit: 1s
Memory Limit: 256MB

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