两个指针, fast和slow之间相差n个节点, fast到最后一个node的时候, slow在要删除的节点的前一个node, 执行删除

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    /**
     * 两个指针, fast和slow之间相差n个节点,
     * fast到最后一个node的时候,
     * slow在要删除的节点的前一个node, 执行删除
     */
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0, head);
        ListNode fast = dummy, slow = dummy;
        for (int i = 0; i < n; i++) {
            fast = fast.next;
        }
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }

        ListNode nxt = slow.next;
        slow.next = nxt.next;

        return dummy.next;
    }
}