链表中间段逆序

class Solution{
public:
    ListNode * reverseBetween(ListNode * head,int m, int n){
    int change_len = n - m +1;//计算需要逆置节点个数
    ListNode *pre_head = NULL; // 初始化开始逆置节点的前驱
    ListNode *result = head; //最终转化后的链表头节点，菲特殊情况即为head
    while(head && --m ){// 将head向前移动m-1位置
        pre_head = head;
        head = head->next;
    }
    ListNode * modify_list_tail = head;//将modify_list_tail 指向当前的head,即逆置后的链表尾
    ListNode * new_head = NULL;
    while(head && change_len){
          ListNode * next = head->next;
          head->next = new_head;
          new_head = head;
          head = next;
          change_len -- ;

    }
    modify_list_tail->next = head;
if(pre_head){
    pre_head->next = new_head;//将逆置 链表开始的节点前驱与逆置后的头节点链接
}
else{
    result = new_head;
}
}

}