题解、测试

//先把m-n反转，再设置mPre nNext的连接
//三个辅助指针用于反转
ListNode node1 = (mPre == null)?head : mPre.next;//指向m
ListNode node2 = node1.next;
ListNode node3 = null;
node1.next = nNext;//m的next指向n的next
//开始反转
while(node2 != nNext){
    node3 = node2.next;
    node2.next = node1;
    node1 = node2;
    node2 = node3;
}//循环结束，node1指向n
//判断是否换头，根据m是否为head
if(mPre == null){
    return node1;
}else{
    mPre.next = node1;
    return head;
}

}