链表指定区间反转

 public ListNode reverseBetween (ListNode head, int m, int n) {
        // write code here
        int i = 1;
        ListNode res = head;
        ListNode pre = new ListNode(1);
        pre.next = head;
        ListNode flag=new ListNode(0);
        while(head!=null){
            if(i>=m && i<=n){
                if(i==m){
        //保存反转后的最后一个节点 便于后面链接
                    flag = pre.next;
                }
                ListNode temp = head.next;
                head.next = pre.next;
                pre.next = head;
                head = temp;
                if(i==n){
        //将反转后的最后一个节点连接起来
                    flag.next = head;
                }
            }else if(i>n){
                head = head.next;
            }else{
                head = head.next;
                pre = pre.next;
            }
            i++;
            //head = head.next;
            //pre = pre.next;
        }
        //**注意可能从第一个节点就开始反转，所以结果也不一样**
        return m==1?pre.next:res;
    }