19. 删除链表的倒数第N个节点

给定一个链表: 1->2->3->4->5, 和 n = 2.

当删除了倒数第二个节点后，链表变为 1->2->3->5.

/**
 * 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) {
        ListNode slow = head;
        ListNode fast = head;
        for (int i = 0; i < n; i++) {
            fast = fast.next;
        }
        if (fast == null) {
            return slow.next;
        }
        while (fast.next != null) {
            slow = slow.next;
            fast = fast.next;
        }
        if (slow.next != null) {
            slow.next = slow.next.next;
        }
        return head;
    }
}