单次循环

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode removeNthFromEnd (ListNode head, int n) {
        // write code here
        ListNode slow = head,fast = head,pre = null;
        if(head.next == null){
            return null;
        }
        while(fast != null){
             fast = fast.next;
             if(n == 0){
                 pre = slow;
                 slow = slow.next;
             }else{
                 n--;
             }

        }
        ListNode current = head;
        if(slow == head){
            return head.next;
        }else{
            pre.next = pre.next.next;
            return head;
        }


    }
}