算法-链表：删除倒数第n个节点

删除链表的倒数第n个节点（n从1开始）。

//方法：采用双指针法，定义fast指针和slow指针同时指向虚拟头节点
//fast先走n+1步，此时fast和slow之间的距离是n+1
//然后slow和fast同时走，当fast指向nullptr时，slow和nullptr之间的距离是n+1，slow指向倒数第n个节点的前一个
ListNode *delN(ListNode *head,int n){
    ListNode *dummyNode = new  ListNode();
    dummyNode->next = head;
    ListNode *slow = dummyNode;
    ListNode *fast = dummyNode;
    while (n--) {
        fast = fast->next;
    }
    fast = fast->next;//第n+1步
    while (fast!=nullptr) {
        fast = fast->next;
        slow = slow->next;
    }
    //删除slow后面的节点
    slow->next = slow->next->next;
    return head;
}