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 *reverseKGroup(ListNode *head, int k) {
        int cnt = 0;
        bool isHead = true;  //是否链头
        ListNode *p = head, *q = NULL, *prev = NULL;
        while(p){
            cnt = 0;
            q = p->next;
            while(q && cnt < k-2){
                q = q->next;
                cnt++;
            }
            if(q == NULL){       //不够K个数
                return head;
            }
            cnt = 0;
            while( cnt < k-1){
                q = p->next;
                p->next = q->next;
                if(isHead){
                    q->next = head;
                    head = q;
                }else{
                    q->next = prev->next;
                    prev->next = q;
                }
                cnt++;
            }
            isHead = false;
            prev = p;
            p = p->next;
        }
        return head;
    }
};