struct ListNode* reverseBetween(struct ListNode* head, int m, int n ) {
    // write code here
    struct ListNode *dum = malloc(sizeof(struct ListNode));
    dum->val = -1;
    dum->next = head;

    struct ListNode *pre = dum;
    for (int i = 0; i < m - 1; i++) {
        pre = pre->next;
    }
    struct ListNode *cur = pre->next;
    struct ListNode *next;
    for (int i = 0; i < n - m; i++) {
        next = cur->next;
        cur->next = next->next;
        next->next = pre->next;
        pre->next = next;
    }
    return dum->next;
}