Can you get the loop ?

• Time : $O(n)$
• Space : $O(1)$

int getLoopSize(Node* startNode)
{
    Node *slow = startNode;
    Node *fast = startNode;
    do {
        slow = slow->getNext();
        fast = fast->getNext()->getNext();
    } while(slow != fast);
    int res = 0;
    Node* runner = slow;
    do {
        runner = runner->getNext();
        res += 1;
    }while(runner != slow);
    return res;
}