反转局部链表

#include <iostream>
using namespace std;

struct ListNode{
    int val;
    ListNode* next;
    ListNode(int v, ListNode* n):val(v), next(n){}
};

ListNode* reverse(ListNode* head) {
    if (head == nullptr) {
        return head;
    }

    ListNode pre(-1, head);
    ListNode* cur = head;
    while (cur->next != nullptr) {
        auto tmp = cur->next;
        cur->next = tmp->next;
        tmp->next = pre.next;
        pre.next = tmp;
    }

    return pre.next;
}

ListNode* reversek(ListNode* head, int k) {
    ListNode pre(-1, head);
    ListNode* first = head;
    ListNode* second = head;
    ListNode* third = &pre;

    int idx = 1;
    while (first != nullptr) {
        idx = 1;
        while (first != nullptr && idx < k) {
            first = first->next;
            idx++;
        }

        if (first == nullptr) {
            break;
        }

        auto tmp = first->next;
        first->next = nullptr;
        third->next = reverse(second);
        second = tmp;
        first = second;
        while (third->next != nullptr) {
            third = third->next;
        }
    }

    third->next = second;
    return pre.next;
}

int main() {
    ListNode* n7 = new ListNode(7, nullptr);
    ListNode* n6 = new ListNode(6, n7);
    ListNode* n5 = new ListNode(5, n6);
    ListNode* n4 = new ListNode(4, n5);
    ListNode* n3 = new ListNode(3, n4);
    ListNode* n2 = new ListNode(2, n3);
    ListNode* n1 = new ListNode(1, n2);
    ListNode* n0 = new ListNode(0, n1);
    ListNode* head = n0;

    int k = 3;
    auto res = reversek(head, k);

    while (res != nullptr) {
        cout << res->val << " ";
        res = res->next;
    }

    cout << endl;

}