Force buckle 25 A group of K flipped linked lists

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        if (head == nullptr)    return head;
        auto tail = head;
        for(int i = 0; i < k; ++i)  // First, let tail Move k Nodes 
        {
            if(tail == nullptr) return head;    // Insufficient k Time 
            tail = tail->next;
        }
        ListNode* cur = head, *pre = nullptr, *next = nullptr;
        while(cur != tail)      // Be careful : reverse[head, tail)
        {
            next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        head->next = reverseKGroup(tail, k);  //@ In this group ,head Became the last node , Note the use of recursion (tail, k)
        return pre;     //@ Return to the flipped head node 
    }
};