/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dumy = new ListNode(0);//建立一个不用的头。。。然后真正的头在next里面,减少对头的处理
        dumy.next = head;
        ListNode p = dumy;
        ListNode q = dumy;
        for(int i = 0 ; i < n ; i++) p = p.next; 
        while(p.next != null){
            p = p.next;
            q = q.next;
        }
        q.next = q.next.next; 
        return dumy.next;
    }
}