Nth to Last Node in List

public class Solution {
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: Nth to last node of a singly linked list. 
     */
    ListNode nthToLast(ListNode head, int n) {
        // write your code here
        if (head == null) return null;
        
        ListNode fast = head, slow = head;
        for (int i = 0; i < n; i++) {
           fast = fast.next; 
        }
        
        while(fast != null) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
        // 3 - 4 - 5 - 6, n = 2
        // result should be 5 not 4
    }
}