решение https://www.codewars.com/kata/52a89c2ea8ddc5547a000863

can_you_get_the_loop.cs

public class Kata
{
  public static int getLoopSize(LoopDetector.Node startNode)
  {
  
    LoopDetector.Node prev = null;
    LoopDetector.Node curr = startNode;
    LoopDetector.Node temp = null;
    
    int n1 = 0;
    int n2 = 0;
    
    do
    {
      temp = curr;
      curr = curr.next;
      temp.next = prev;
      prev = temp;
      n1++;
    }  while (curr != null);
    
    curr = startNode;
    
    do
    {
      temp = curr;
      curr = curr.next;
      temp.next = null;
      n2++;
    } while (curr.next != null);
    
    return  2 * n2 - n1 + 1;
  }
}