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题目描述 有一棵n个点的树和两个整数p, q，求满足以下条件的四元组(a, b, c, d)的个数： 1.$1\leq a,b,c,d \leq n$ 2.点a到点b的经过的边数为p。 3.点c到点d的经过的边数为q。 4.不存在一个点，它既在点a到点b的路径上，又在点c到点d的路径上。 输入描述: 第一行三个整数n，p，q。 接下来n - 1行，每行两个整数u, v，表示树上存在一个连接点u和点v的边。 输出描述: 输出一个整数，表示答案。 示例1 输入 5 2 1 1 2 2 3 3 4 2 5 输出 4 说明 合法的四元组一共有: (1, 5, 3, 4)， (1, 5, 4, 3)， (5, 1, 3 ,4)， (5, 1, 4, 3)。 示例2 输入 4 1 1 1 2 2 3 3 4 输出 8 备注: 对于前20%的数据，n,p,q≤50。 对于前40%的数据，n,p,q≤200。 对于另外10%的数据，p = 2, q = 2。 对于另外10%的数据，树是一条链。 对于另外10%的数据，树随机生成。 对于所有数据1≤n,p,q≤3000，1≤u,v≤n,保证给出的是一棵合法的树。 分析 我已经弱到连$n^2$枚举路径都不会了 再一次求助Master_Yi 这个题只要理顺了就挺好想的了（说得好像我想得出来似的。 由于不相交的情况不好求，所直接看相交的情况。 找规律

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: $rows = Yii::app()->db->createCommand()->select('*')->from('states')->queryAll(); above code returns only single first row, while according to documentation it should return all rows. 回答1: queryAll() returns an array and u need to iterate in order to get all the records foreach($rows as $row){ //do something } 文章来源: Yii::app()->db->createCommand($sql1)->queryAll() is not returning whole table [closed]

数位dp是以数位上的关系为转移关系而进行的一种计数dp，题目基本类型是给定区间[l ,r] ，求l到r之间满足要求的数字的个数 . dp状态的转移方式通常是用 递归+记忆化搜索 ，转移顺序一般是由高数位转移到底数位 ，其中就是记忆化搜索保证了数位dp的高效率 例如千位2到百位转移要枚举0,1,2,3 ...(2000,2100,2200,2300...) ,而千位3也是同样的（3000，3100，3200,3300...）,其进行的都是对三位数000~999的统计,所以低位统计过程只用进行一次就可将结果应用于所有高位状态上，减少了重复过程的进行. 结果的输出形式是 0~r 之间的dp 与 0~l之间的dp 进行相减 来求 l到r 之间的 dp. printf("%lld\n",solve(r)-solve(l-1)); 值得注意的点是边界 l和r 不能进行记忆化搜索 ，比如 dp[2][sta] 记录的是 000~999（三位数） 中满足条件的数字的个数 ，而对于l = 2250 ，其在2000之后的三位数只有 100~250 ，所以这时候如果直接记忆化返回 dp[2][sta] 就会出现多记. 有的题目对前导零有要求，有的没有，做的时候随机应变。 例题： HDU 2089 不要62 题解：转移状态很清晰明了的题目，主要通过此题了解 递归+记忆化的转移方式

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 由 翻译 强力驱动 问题: I want to use multiple database engines with a single sqlalchemy database model. Following situation: I have a photo album software (python) and the different albums are stored in different folders. In each folder is a separate sqlite database with additional information about the photos. I don't want to use a single global database because with this way I can simply move, delete and copy albums on a folder base. Opening a single album is fairly straightforward: Creating a db session: maker = sessionmaker ( autoflush = True ,

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I am quite a beginner when it comes to Oracle. I am having trouble figuring out how to do something similar to this : SELECT ID, NAME, TO_CHAR(DATEBIRTH, 'DD/MM/YYYY HH24:MI:SS') FROM PEOPLE WHERE DATEBIRTH >= ANOTHERDATE - NDAY To put it short, I want to select everyone who were born N days before a specific date and time but I am not quite sure that this is the way to do it nor that it would give me the results I expect. PS: I am developping under oracle8i. 回答1: Your query looks correct to me. That's how you subtract days from dates in

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 由 翻译 强力驱动 问题: I have a column named Lastmodified , with a data type of Date , but it should have been DateTime . Is there any way of converting the column? When I use the 'Design' feature of SQL Server Management Studio I get the following error: Saving changes is not permitted. The changes you have made require the following table to be dropped and re-created. Not really interested in dropping the table, I just want to know if it is possible to convert a column from Date to Datetime or do I have to delete the column and create a new one with

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have this query which works but when I try to write the equivalent in LINQ I get the incorrect SQL produced. My query is: SELECT COUNT(*) FROM tableName GROUP BY ColumnId I've tried writing it as: tableName.GroupBy(x => x.ColumnId).Count() But looking in LINQPad it is producing the SQL: SELECT COUNT(*) AS [value] FROM ( SELECT NULL AS [EMPTY] FROM [tableName] AS [t0] GROUP BY [t0].[ColumnId] ) AS [t1] What am I doing wrong? Thanks! 回答1: Your LINQ query is counting the number of groups but your SQL query is producing the counts by group.

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可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I've studied "Simulating Ocean Water" article by Jerry Tessendorf and tried to program the Statistical Wave Model but I didn't get correct result and I don't understand why. In my program I tried only to create a wave height field at time t = 0 without any further changes in time. After execution of my program I got not what I was expecting: Here's my source code: clear all; close all; clc; rng(11); % setting seed for random numbers meshSize = 64; % field size windDir = [1, 0]; % ||windDir|| = 1 patchSize = 64; A = 1e+4; g = 9.81; %

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I am migrating a portal to Flask with Flask-SQLAlchemy (MySQL). Below is the code I used to create my DB for my existing portal: Users = """CREATE TABLE Users( id INT UNSIGNED AUTO_INCREMENT NOT NULL, UserName VARCHAR(40) NOT NULL, FirstName VARCHAR(40) NOT NULL, LastName VARCHAR(40) NOT NULL, EmailAddress VARCHAR(255) NOT NULL, Password VARCHAR(40) NOT NULL, PRIMARY KEY (id) ) """ Here is how I am trying to use it in SQLAlchemy: class Users(db.Model): id = db.Column(db.Integer, primary_key=True) UserName = db.Column(db.String(40)) FirstName