leecode19.删除链表的倒数第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) {
        if(head==null||head.next==null){
            return null;
        }
        ListNode p=head;
        ListNode q=head;
        for(int i=0;i<n;i++){
            p=p.next;
        }
        if(p==null){
            return head.next;
        }
        while(p.next!=null){
            p=p.next;
            q=q.next;
        }
        q.next=q.next.next;
        return head;
    }
}