K 个一组翻转链表(LeetCode困难)(链表)

K 个一组翻转链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        auto dummy = new ListNode(-1);
        dummy->next = head;
        for(auto p = dummy; ;){
            auto q = p;
            for(int i = 0; i < k && q; ++i) q = q->next;
            if(!q) break;
            //内部翻转
            auto a = p->next, b = p->next->next;
            for(int i = 0; i < k - 1; ++i){
                auto c = b->next;
                b->next = a;
                a = b, b = c;
            }
            auto c = p->next;
            p->next = a; c->next = b;
            p = c;
        }

        return dummy->next;
    }
};