面试题24：单链表逆序

ListNode* reverseList(ListNode* head) {
    if (head == nullptr || head->next == nullptr) return head;
    ListNode* dummy = new ListNode(-1);
    dummy->next = head;

    ListNode* p = head->next;
    ListNode* p_pre = head;
    while (p) {
        ListNode* p_next = p->next;
        p_pre->next = p->next;
        p->next = dummy->next;
        dummy->next = p;
        p = p_next;
    }
    return dummy->next;

}

ListNode *reverseBetween(ListNode *head, int m, int n) {
    if (n - m < 1) return head;
    ListNode* dummy = new ListNode(-1);
    dummy->next = head;

    ListNode *start = head;
    ListNode* start_pre = dummy;
    for (int i = 1; i < m; i++) {
        start_pre = start;
        start = start->next;

    }
    for (int i = 0; i < n - m; i++) {
        ListNode* tmp = start->next;
        start->next = start->next->next;
        tmp->next = start_pre->next;
        start_pre->next = tmp;
    }
    return dummy->next;
}