反转链表 II，反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

反转链表 II：
反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}
};

ListNode* reverseBetween(ListNode* head, int m, int n) {
    if (head == nullptr || m >= n) return head;
    ListNode dummy(0);
    dummy.next = head;
    ListNode* prev_m = &dummy;
    ListNode* prev_n = &dummy;

    int count_m = m - 1;
    int count_n = n;
    while (count_m && prev_m != nullptr) {
        prev_m = prev_m->next;
        count_m--;
    }
    if (!prev_m || !prev_m->next) {
        return head;
    }
    while (count_n && prev_n != nullptr) {
        prev_n = prev_n->next;
        count_n--;
    }

    ListNode* first = prev_m->next;
    ListNode* last_next = prev_n->next;
    ListNode* pre = last_next;
    while (first != last_next) {
        ListNode* next = first->next;
        first->next = pre;
        pre = first;
        first = next;
    }
    prev_m->next = pre;
    return dummy.next;
}

int main() {
    ListNode* a = new ListNode(1);
    ListNode* b = new ListNode(2);
    ListNode* c = new ListNode(3);
    ListNode* d = new ListNode(4);
    ListNode* f = new ListNode(5);
    ListNode* g = new ListNode(6);
    a->next = b;
    b->next = c;
    c->next = d;
    d->next = f;
    f->next = g;
    g->next = nullptr;

    ListNode* result = reverseBetween(a, 3, 5);
    while (result) {
        cout << result->val << " ";
        result = result->next;
    }
    return 0;
}