/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *dummy=new ListNode(-1);//虚拟头节点
        dummy->next=head;//虚拟头节点指向头节点
        ListNode *first=dummy,*second=dummy;  //定义两个指针  初始时都指向虚拟节点
        for(int i=0;i<n;i++)   first=first->next;//循环结束后first指向第n-1个节点
        while(first->next)
        {
            first=first->next;
            second=second->next;
        }
        second->next=second->next->next;//删除（skip）
        return dummy->next;
    }
};