计算链表长度

/**
 * 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) {
        auto dummy=new ListNode(-1);
        dummy->next=head;
        int sum=0;
        auto p=dummy;
        while(p->next)sum++,p=p->next;  //结点总数
        p=dummy;
        for(int i=0;i<sum-n;i++)p=p->next;
        p->next=p->next->next;
        return dummy->next;
    }
};
//链表