删除链表倒数第n个节点

/**
 * 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 dummy=new ListNode(0);
        dummy.next=head;
        ListNode first=head;
        
        int lenght=0;
        while(first!=null)
        {
            first=first.next;
            lenght++;
        }
        
        lenght=lenght-n;
        first=dummy;
        while(lenght>0)
        {
            lenght--;
            first=first.next;
        }
        first.next=first.next.next;
        
        return dummy.next;
    }
}