居然把头插法写错了，debug了一个多小时/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* reverseKGroup(ListNode* head, int k) {        if(k == 1 || head == nullptr)  return head;        ListNode *tail = head;        int i;        for(i=0;i<k && tail != nullptr;i++,tail=tail->next);        if(i<k) return head;        tail = reverseKGroup(tail, k);        ListNode* rear = head,*tmp=nullptr;        head= tail;        for(int i = 0;i<k;i++){              tmp = rear->next;                      rear->next = head;              head = rear;              rear = tmp;        }        return head;    }};