Leetcode 61 -- Rotate List

public ListNode rotateRight(ListNode head, int k) {
        if (head == null) {
            return null;
        }
        
        ListNode before = head;
        ListNode after = head;
        
        // get length 
        int length = 0;
        while(after != null) {
            length++;
            after = after.next;
        }
    
        // get valid k less than length
        k = k % length;
        
        after = head;
        for(int i=0; i<k; i++) {
            after = after.next;
        }
        
        while (after.next != null) {
            before = before.next;
            after = after.next;
        }
        
        after.next = head;
        head = before.next;
        before.next = null;
        return head;
    }