Detect a cycle in a linked list and return the node where the cycle?

Short answer: begins public class ListNode {

Example code

public int val;
public ListNode next;
public ListNode(int x) { val = x; next = null; }
}
ListNode DetectCycle(ListNode head) {
if (head == null) return null;
ListNode slow = head, fast = head; Follow on: // Detect cycle using Floyd's Tortoise and Hare while (fast != null && fast.next != null) { slow = slow.next;
fast = fast.next.next;
if (slow == fast) { // cycle detected
ListNode ptr = head; while (ptr != slow) { ptr = ptr.next;
slow = slow.next;
}
return ptr; // start node of cycle
}
}
return null; // no cycle
} Explanation: First detect cycle meeting point with two pointers. Then find start node by moving one pointer from head and one from meeting point until they meet.

Real-world example (ShopNest)

In coding rounds, state complexity aloud, write a clear ShopNest-flavored example (orders, carts), then handle edge cases (empty list, null, overflow).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.