Rotate List

class Solution
{
    public ListNode rotateRight(ListNode head, int k)
    {
        int size = len(head);
        if (size == 0) return null;
        k %= size;
        if (k == 0) return head;
       
        ListNode fast = head;
        while (k!= 0)
        {
            k--;
            fast = fast.next;
        }
       
        ListNode slow = head;
        while (fast.next != null)
        {
            fast = fast.next;
            slow = slow.next;
        }
       
        ListNode result = slow.next;
        slow.next = null;
        fast.next = head;
        return result;
    }
   
   
    int len(ListNode head)
    {
        int count = 0;
        while (head != null)
        {
            ++count;
            head = head.next;
        }
        return count;
    }
}

Input
head =
[1,2,3,4,5]
k =
2
Output
[4,5,1,2,3]
Expected
[4,5,1,2,3]