【LBLD】如何K个一组反转链表

【LBLD】如何K个一组反转链表

/**
 * 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* reverseN(ListNode* head, int n) {
        ListNode *pre = nullptr;
        ListNode *cur = head;
        ListNode *nxt = head;
        for (int i = n; i > 0; i--) {
            nxt = cur->next;
            cur->next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }

    ListNode* reverseKGroup(ListNode* head, int k) {
        if (head == nullptr) {
            return head;
        }

        ListNode *a = head;
        ListNode *b = head;
        for (int i = 0; i < k; i++) {
            if (b == nullptr) return head;
            b = b->next;
        }

        head = reverseN(a, k);
        a->next = reverseKGroup(b, k);
        return head;
    }
};