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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (head == NULL) return NULL;
        
        ListNode* last = new ListNode(0);
        last->next = head;
        head = last;
        while(last->next != NULL)
        {
            // p1,p2 
            ListNode* p1 = last->next;
            ListNode* p2 = p1;
            int count = 1;
            while (count < k && p2->next != NULL)
            {
                p2 = p2->next;
                ++count;
            }
            if (count == k)
            {
                // p0 
                ListNode* p0 = p1;
                p2 = p1->next;
                while(count-- > 1)
                {
                    ListNode* tmp = p2->next;
                    p2->next = p1;
                    p1 = p2;
                    p2 = tmp;
                }
                last->next = p1;
                p0->next = p2;
                last = p0;
            }
            else
            {
                break;
            }
        }
        return head->next;
    }
};