92. Reverse Linked List II

class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        ListNode node1 = head;
//        int len = 0;             //可以不用加
//        while(node1 != null) {
//            len++;
//            node1 = node1.next;
//        }
//        if(len == 1) return head;
//        node1 = head;
        ListNode pre = null;
        for(int i = 1; i <= m - 1; i++) {
            pre = node1;
            node1 = node1.next;    
        }
        ListNode last = head;
        for(int i = 1; i <= n; i++) {
            last = last.next;
        }
        node1 = head;
        ListNode record = node1;
        int count = 1;
        while(count <= n) {
            if(count >= m) {
                record = node1.next;
                node1.next = last;
                last = node1;
                node1 = record;
                if(count == n) {
                    if(pre != null) {  //注意要判断pre是不是null
                        pre.next = last;
                    }else {
                        head = last;
                    }    
                }
            }else {
                node1 = node1.next;
            }
            count++;
        }
        return head;
        
        
    }
}