旋转链表（中等）
给定一个链表，旋转链表，将链表每个节点向右移动 k 个位置，其中 k 是非负数。
#include<stdio.h>
struct ListNode
{int val;ListNode *next;ListNode(int x): val(x), next(NULL) {}
};
class Solution
{
public:ListNode* rotateRight(ListNode *head, int k){if(head == NULL) return NULL;int n=0;for(auto p=head; p; p=p->next) n++;k %= n;auto first = head, second = head;while(k--)first = first->next;while(first->next){first = first->next;second = second->next;}first->next = head;head = second->next;second->next = NULL;return head;}
};