[LeetCode] Reverse Nodes in k-Group

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    
    ListNode *head0;
    int flag ;
    ListNode *reverseKGroup(ListNode *head, int k) {
        if(k<2)
            return head;
         ListNode *h = reverse(head,k);
         if(flag ==0 )
             return head;
        
         ListNode *tail = head;
         while(head0 != NULL && flag==1){
             ListNode *tmp = head0;
             tail->next = reverse(head0,k);
             tail = tmp;
         }

         return h;
    }
private:
    ListNode *reverse(ListNode *head, int n){
        ListNode *p = head;
        int num = n;
        while(num>0 && p!=NULL){
            p = p->next;
            num--;
        }
        if(p == NULL && num>0){
            flag = 0;//flag == 0表示未翻转
            return head;
        }
            
        flag = 1; //flag == 1表示翻转
        ListNode *p1 = head,*p2=p1->next,*p3; 
        while(n-1){
           p3 = p2->next;
           p2->next = p1;
           p1 = p2;
           p2 = p3;
           n--;
        }
        head->next = NULL;
        head0 = p2;
        return p1;
    
    }
};