19. Remove Nth Node From End of List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        
        // dummy  ->  1  ->  null
        // slow (0)
        //         fast(1) 
        
        // dummy  ->  1  ->  2  -> 3  ->  4  ->  5  -> null
        // slow(0)
        //         fast(2)
        //                           slow(0)
        //                                             fast(2)
        
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode slow = dummy;
        ListNode fast = dummy;
        for (int i = 0; i < n; i++) {
            fast = fast.next;
        }
        while (fast != null && fast.next != null) {  // we should also have fast != null
            slow = slow.next;
            fast = fast.next;
        }
        slow.next = slow.next.next;
        return dummy.next;
    }
}