Day60.java

class ListNode {
    int val;
    ListNode next;

    ListNode(int val) {
        this.val = val;
        this.next = null;
    }
}

public class Day60 {
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) {
            return null;
        }

        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                break;
            }
        }

        if (fast == null || fast.next == null) {
            return null;
        }


        slow = head;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }

        return slow;
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = head.next; // Creating a cycle

        LinkedListCycle solution = new LinkedListCycle();
        ListNode cycleStart = solution.detectCycle(head);
        if (cycleStart != null) {
            System.out.println("Sample Output: " + cycleStart.val);
        } else {
            System.out.println("Sample Output: NULL");
        }
    }
}