K个一组翻转链表

面试的时候居然写炸了。补一下

#include <iostream>

using namespace std;

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:
    std::pair<ListNode *, ListNode *> reverse(ListNode *start, ListNode *end) {
        cout << "reverse(" << start->val << ", " << end->val << ")" << endl;
        ListNode *pre = end->next, *pos = start, *next = start->next;
        auto x = end->next;
        while (pos != x) {
            next = pos->next;
            pos->next = pre;
            pre = pos;
            pos = next;
        }
        return { end, start };
    }

    ListNode* reverseKGroup(ListNode* head, int k) {
        int n = 0;
        ListNode *res = nullptr, *p = head, *start = head, *tmp = nullptr;
        while (p != nullptr) {
            n++;
            if (n == k) {
                n = 0;
                auto se = reverse(start, p);
                p = start = se.second->next;
                if (tmp != nullptr) {
                    tmp->next = se.first;
                }
                tmp = se.second;
                if (res == nullptr) res = se.first;
            }
            else {
                p = p->next;
            }
        }
        return res;
    }
};

void print(ListNode *p)
{
    while (p != nullptr) {
        cout << p->val << ' ';
        p = p->next;
    }
    cout << "NULL " << endl;
}

int main()
{
    ListNode *node = new ListNode(1);
    ListNode *node1 = new ListNode(2);
    ListNode *node2 = new ListNode(3);
    ListNode *node3 = new ListNode(4);
    ListNode *node4 = new ListNode(5);
    ListNode *node5 = new ListNode(6);

    node->next = node1;
    node1->next = node2;
    node2->next = node3;
    node3->next = node4;
    node4->next = node5;
    node5->next = nullptr;
    
    Solution s;

    print(s.reverseKGroup(node, 2));

    int xyz;
    cin >> xyz;

    return 0;
}