problem solving online judge

The 10 Most Popular Coding Challenge Websites [Updated for 2021]

A great way to improve your skills when learning to code is by solving coding challenges. Solving different types of challenges and puzzles can help you become a better problem solver, learn the intricacies of a programming language, prepare for job interviews, learn new algorithms, and more.

Below is a list of some popular coding challenge websites with a short description of what each one offers.

1. TopCoder

TopCoder is one of the original platforms for competitive programming online. It provides a list of algorithmic challenges from the past that you can complete on your own directly online using their code editor. Their popular Single Round Matches are offered a few times per month at a specific time where you compete against others to solve challenges the fastest with the best score.

The top ranked users on TopCoder are very good competitive programmers and regularly compete in programming competitions. The top ranked user maintains his own blog titled Algorithms weekly by Petr Mitrichev where he writes about coding competitions, algorithms, math, and more.

2. Coderbyte

Coderbyte provides 200+ coding challenges you can solve directly online in one of 10 programming languages (check out this example ). The challenges range from easy (finding the largest word in a string) to hard (print the maximum cardinality matching of a graph).

They also provide a collection of algorithm tutorials , introductory videos, and interview preparation courses . Unlike HackerRank and other similar websites, you are able to view the solutions other users provide for any challenge aside from the official solutions posted by Coderbyte.

3. Project Euler

Project Euler provides a large collection of challenges in the domain of computer science and mathematics. The challenges typically involve writing a small program to figure out the solution to a clever mathematical formula or equation, such as finding the sum of digits of all numbers preceding each number in a series.

You cannot directly code on the website in an editor, so you would need to write a solution on your own computer and then provide the solution on their website.

4. HackerRank

HackerRank provides challenges for several different domains such as Algorithms, Mathematics, SQL, Functional Programming, AI, and more. You can solve all the challenge directly online (check out this example ).

They provide a discussion and leaderboard for every challenge, and most challenges come with an editorial that explains more about the challenge and how to approach it to come up with a solution.

Currently, if you don't solve the problem, then you can't see the solution of others. If you also try to check the editorial before solving the problem, then you won't get the point for solving the problem at all.

As an example, here I haven't solved the problem, and I am trying to check others' submissions:

And here, I haven't solved the problem, and I am trying to check the editorial:

HackerRank also provides the ability for users to submit applications and apply to jobs by solving company-sponsored coding challenges.

5. CodeChef

CodeChef is an Indian-based competitive programming website that provides hundreds of challenges. You are able to write code in their online editor and view a collections of challenges that are separated into different categories depending on your skill level (check out this example ). They have a large community of coders that contribute to the forums, write tutorials , and take part in CodeChef’s coding competitions .

6. Exercism.io

Exercism is a coding challenge website that offers 3100+ challenges spanning 52 different programming languages. After picking a language that you'd like to master, you tackle the coding challenges right on your machine (Exercism has their own command line interface that you can download from GitHub).

It is a bit different from other challenge websites, however, because you work with a mentor after completing each challenge. The mentor reviews your answers online and helps you improve them if needed. Once your answers have been approved and submitted, you unlock more challenges.

7. Codewars

Codewars provides a large collection of coding challenges submitted and edited by their own community. You can solve the challenges directly online in their editor in one of several languages. You can view a discussion for each challenges as well as user solutions.

8. LeetCode

LeetCode is a popular Online Judge that provides a list of 190+ challenges that can help you prepare for technical job interviews. You can solve the challenges directly online in one of 9 programming languages. You are not able to view other users' solutions, but you are provided statistics for your own solutions such as how fast your code ran when compared to other users' code.

They also have a Mock Interview section that is specifically for job interview preparation, they host their own coding contests , and they have a section for articles to help you better understand certain problems.

Sphere Online Judge (SPOJ) is an online judge that provides over 20k coding challenges. You are able to submit your code in an online editor . SPOJ also hosts their own contests and has an area for users to discuss coding challenges. They do not currently provide any official solutions or editorials like some other websites do, though.

10. CodinGame

CodinGame is a bit different from the other websites, because instead of simply solving coding challenges in an editor, you actually take part in writing the code for games that you play directly online. You can see a list of games currently offered here and an example of one here . The game comes with a problem description, test cases, and an editor where you can write your code in one of 20+ programming languages.

Although this website is different than typical competitive programming websites such as the ones mentioned above, it is still popular amongst programmers who enjoy solving challenges and taking part in contests.

This list was based on a few things: my own experiences using the websites, some Google searches , Quora posts , and articles such as this one and this one . I also frequented some forums and subreddits such as r/learnprogramming to see what websites were usually recommended by the users there. Disclaimer: I work at Coderbyte which is one of the websites mentioned above.

CEO & Founder at Coderbyte.

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online-judge-solutions

Here are 36 public repositories matching this topic..., risal-shefin / competitive-programming.

Competitive Programming Stuff

• Updated Jan 21, 2024

khalid586 / Competitive-programming-Topics

This repository contains the most common topics found in competitive programming . I have tried to combine all my practiced problems in this repository.

• Updated Oct 23, 2023

Mousa-mahmood-hilal / solution-UVA-in-python

UVA solution

• Updated Mar 15, 2023

jromero132 / online-judges-problem-solutions

My solutions for competitive programming problems on several platforms.

• Updated Feb 9, 2023

asad14053 / My-Online-Judge-Solution

Alogrithm,Data Structure, Problem solving, Online Judge Solution

• Updated Aug 6, 2019

shiningflash / Dynamic-Programming

Here are some dynamic programming problem solutions.

• Updated Jan 24, 2021

gpwaob92679 / JudgeGirl-solution

Solution code and helper scripts for the JudgeGirl online judge.

• Updated Dec 21, 2023

akash1518 / Online-Judge-Solutions

It contain codes for various Online Judge - Code Chef, Spoj, Hacker Rank,Hackerearth

• Updated Aug 20, 2017

sherif17 / Problem_Solving_PST

This repo includes solutions for assigned sheets & contests during problem solving boot camp training. For each problem,there is a read-me file that contains problem statement, constrains & samples for expected input & output.

• Updated Oct 2, 2022

shiningflash / Codeforces

username: amirulislam [codeforces.com/profile/Amirulislam]

aratheunseen / online-judge-solutions

Hackerrank and beecrowd or URI Online Judge Solutions.

• Updated Feb 20, 2022

HasnathJami / Problem-Solving

Here, all of my solved problems are listed according to different categories.

• Updated Nov 18, 2022

Chang9601 / online-judge

온라인 채점 사이트 문제 해답

Tholes / Competitive-programming

My solutions from different online Judge.

• Updated Apr 18, 2020

WEGFan / Algorithm-Contest-Code

My solutions of problems from algorithm contests and online judge

• Updated Dec 19, 2019

ihjohny / UVA-Solutions

Solution of UVA ( https://uva.onlinejudge.org ) problems solved so far

• Updated Dec 14, 2019

shiningflash / Online-Judge-Solutions

Here are my online judges solutions.

• Updated Mar 31, 2021

audacioustux / oj_solutions

Aggregate of all my Online-Judge(Leetcode,CSES,AdventOfCode,Hackerrank, etc.) Solutions Repos

• Updated Dec 25, 2023

mahmudul-hasan-sreejon / computer-programming-boi-solutions

• Updated Apr 3, 2017

joeyTurnerr / VirtualJudge-Walkthrough-cpp

website: https://vjudge.net/

• Updated Apr 22, 2022

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How An Online Judge Works And How To Avoid Time Limit Exceeded Problem?

In this article, you will get consent on how an online judge works and this article discusses very frustrating error Time Limit Exceed error that coders get at some point in time while solving the questions at online platforms. The article primarily discusses three things:

• How does an online judge work?
• Why do we get TLE?
• How to avoid TLE?

How Does An Online Judge Work?

There are various online judges Hackerrank, HackerEarth, Codechef. All of the have their respective algorithms and systems of evaluating submissions. Let’s consider an online judge who is accepting the submission for a given problem. For this submission, the judge has some input and some output files loaded in it already. The judge passes the submission to the processor and this processor has some processing limits. Let’s say in 1 sec the processor is able to perform 10^8 operations. Now, what happens internally when you submit your program is that your program executes and it gets the input from the processor and verifies the output with already loaded test cases in it. If it matches then it is a case of a correct answer but sometimes during the execution, if the program takes more time than the required time limit then it prompts the user time limit exceeded error. For example, if the required limit was 1 second and your program were taking more than 1 second for execution, then your judge will issue a kill command and your output will be killed and in the output, you will get Time Limit Exceeded (Optimize Your Code) . This is how an online judge works.

Why Do We Get TLE?

When your algorithm doesn’t have the required efficiency you get TLE so the idea is to complete the processing in a finite amount of time. Let’s assume you are calculating whether a number is prime or not and the number is of the order 10^18. If you are using the O(n) algorithm you are bound to get TLE and if you use the O(log N) algorithm you won’t get TLE. Below are some common reasons for TLE error…

• Online judges put some restrictions on time and it doesn’t allow you to process your instruction after that time limit. If you take more time than the time limit is specified then you will get the TLE error.
• TLE error also depends on the server architecture, operating system, and the complexity of an algorithm. For different platform server architectures are different and the speed of the execution for the code varies on each server.
• If the programmer is using a slow method of reading and writing the input in the code then it will give you a TLE error.

How To Avoid TLE?

1. Analyze the constraints: If the time limit is 1 sec, your processor is able to execute 10^8 operations.

2. Choose faster input and output method. For example:

• Use buffer reader in java, do not use Scanner
• In C++ use scanf/printf instead of cin/cout,
• Use these two statements in python for speeding up your execution

3. Your program must not contain 4 nested loops if N >=100.

4. Sometimes too many nested loops can make your program slower. So it’s better to optimize your code and reduce the number of loops according to the instruction of input bound already specified in the question. So read the bounds in your code carefully and according to that write your program.

5. Optimize your algorithm or try to find a different solution for the problem statement using another data structure. As the saying goes, experience is the best teacher. The more you code the algorithms, the more efficient the geek you become in coding.

Important Link: How to overcome Time Limit Exceed(TLE)?

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• Competitive Programming

Improve your Coding Skills with Practice

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How to Build Your Own Online Judge System

Introduction.

First, let’s briefly introduce what an Online Judge (OJ) system is. Simply put, it is a system like LeetCode that allows you to submit code for problem-solving and then lets the system check it and give you the final result. Below is a screenshot of LeetCode:

Before LeetCode became popular, the most well-known OJ was probably UVa Online Judge , also known as ACM. In Taiwan, ZeroJudge is more famous.

If you happen to have a need and want to build your own OJ, what should you do?

Open Source OJ Systems

A search on the internet will reveal several open-source OJ systems, among which the following three have more stars and appear to be more stable:

This system appears to have the most complete and feature-rich functionality, and supports the most languages, up to 60 or so! It also supports third-party logins such as Google, Facebook, and Github. The backend is written in Python and is continuously maintained, with fairly complete documentation .

The only drawback is that the interface is a bit plain and not as appealing.

This system was developed by the Nanjing University of Posts and Telecommunications in China and is written in Laravel. The interface uses Material UI and looks more modern, but the documentation is less complete.

This system was developed by Qingdao University in China. The backend is written in Python + Django, the frontend is in Vue, and it is deployed using Docker, making it simple and fast. The supported programming languages are C, C++, Java, and Python. The interface uses Ant Design.

Which one to choose depends on your needs. If the documentation provided on GitHub is complete, just follow the instructions. If it is incomplete, you can ask questions through Issues. You don’t need to worry too much if you don’t speak English well, as these three repositories can be communicated in Chinese.

I chose the last one, the OJ system open-sourced by Qingdao University, because I really like the interface and it is the easiest to deploy among the three.

The deployment process is here: https://github.com/QingdaoU/OnlineJudgeDeploy/tree/2.0 . Since it is deployed using Docker, it is really easy. Basically, just download the docker-compose.yml file and run a command to finish it.

Let’s take a look at the contents of docker-compose.yml:

You can see that it is divided into four services: Redis, Postgres, judge-server, and oj-backend.

However, I had one more requirement at the time, which was to support JavaScript. To achieve this goal, it was not enough to just deploy it. I also had to study how it worked.

Further Study of QDUOJ

First, let’s take a look at the architecture of this system. According to the documentation on GitHub, it is divided into several modules:

• Backend(Django): https://github.com/QingdaoU/OnlineJudge
• Frontend(Vue): https://github.com/QingdaoU/OnlineJudgeFE
• Judger Sandbox(Seccomp): https://github.com/QingdaoU/Judger
• JudgeServer(A wrapper for Judger): https://github.com/QingdaoU/JudgeServer

The issue that we are most concerned about (how to add a new language) has already been raised in an issue: How to add more language support, such as Ruby .

It is mentioned that modifying this file is enough: https://github.com/QingdaoU/OnlineJudge/blob/master/judge/languages.py .

This file is a configuration file, and you can guess what the Judge Server will do here. Here, the configuration of each language will have compile_command and command . The former is used to obtain the compilation command, and the latter is used to obtain the command to run the program. Since the input and output of this OJ are all through stdin / stdout , when you want to add a new programming language, you just need to tell the system how to execute it.

On the contrary, some OJs use functions to fill in the blanks, such as LeetCode mentioned at the beginning. At this time, if you want to add a new language, it will be more troublesome because you need to provide a function template additionally.

In theory, we just need to add such settings:

But if you run it like this, you will find a problem. Someone has already reported it: problem with adding js to language configs . The solution is to set seccomp_rule to None .

What is seccomp ? This is related to the principle of OJ! You can think carefully about the most important question in OJ:

How to safely execute user-submitted code?

If you don’t know what this question is asking, you can imagine the following situations:

• What if someone writes a code that restarts the computer?
• What if someone writes an infinite loop?
• What if someone writes a program that sends the host account password to an external server?

From this, it can be seen that executing code is not that simple, and this is also the core part of OJ.

The QDUOJ Judger source code is here: https://github.com/QingdaoU/Judger/tree/newnew/src .

It is written in C, forks a new process, sets some rules, and then uses execve to execute the command. In the code , it can also be seen that seccomp is used to prevent the content mentioned above.

In short, QDUOJ is well-layered, and the execution process is roughly as follows:

• Enter the front-end page made by Vue.
• Submit the code and call the back-end API (Python).
• The back-end API then calls the Judge Server API (Go).
• The Judge Server API calls the Judger to execute the command (C, execve + seccomp execution).

So each project is responsible for its own tasks.

Going back to the issue of adding JavaScript mentioned earlier, even if seccomp_rule is set to None , there will still be errors when executing JavaScript. After studying for a day or two, I found that the problem was that the memory limit of the problem was too small. I guess that Node.js will consume more memory when it is executed, so as long as the memory is increased (for example, to 1024MB), it will be solved.

However, it is not over yet. There is one last problem, which is that the Node.js version on Ubuntu 16.04 is quite old, and a new version is needed to use ES6 syntax. The solution is to modify the Dockerfile of the JudgeServer and add a command to install the new version of Node.js.

After all the changes are made, you can deploy your own version! Just build the docker image first, and then modify the docker-compose file that we operated at the beginning.

In summary, if you don’t want to make any changes and just want to deploy, I highly recommend QDUOJ. It’s easy to deploy and has a nice interface.

Writing your own OJ

I once tried to write my own OJ, but it was a very basic version: https://lidemy-oj.netlify.com/problems

At that time, I didn’t think of using Linux commands to run it. Instead, I found the library VM2 and thought it would be useful, so I had the idea of writing this simple OJ.

This simple OJ only supports JavaScript and uses the function writing method like LeetCode instead of standard input and output. I spent some time writing the prototype of the Judger:

The key code is addWrapper and runTest . In addWrapper , the function code passed in is executed, and the result is compared with the output to return true or false, indicating whether the match is successful or failed, and whether the answer is correct.

Then problemSchema is the format of the problem, which needs to have a funcName and testCases , and each test case has an input and output . With the above code, a super simple JS function-based Judger can be implemented.

However, this Judger has many shortcomings and cannot be compared with the execution method mentioned above.

Later, when I was researching, I found some good open-source solutions that can be referred to if someone wants to write their own OJ in the future.

The first is the sandbox isolate open-sourced by IOI, which can safely execute commands.

The second is even more amazing, it directly gives you the Judge API, and it’s free: Judge0 API . As long as you pass in the input according to its format, it will tell you the judging result, so you don’t even need to make your own Judge Server.

I wanted to build an OJ before, so I looked for a lot of information, and the biggest problem I encountered was: “I want an OJ that supports JavaScript”, because many of them don’t support it. Later, I had to write my own, which is the little toy written in JS mentioned above. Although it is still usable, many functions are incomplete, and it only supports the simplest answering.

Until January of this year, I wanted to create a real OJ. I originally considered whether to write it myself, but later thought it was too troublesome and decided to find an existing one to modify and add JS support. Although I still encountered some problems, fortunately, I succeeded in the end.

The final result is here: https://oj.lidemy.com/

This OJ is designed to accompany my latest free online course: [ALG101] Don’t rush to write LeetCode , which is a course for beginners. I hope to lay a solid foundation through a series of simple problems and cultivate programming thinking ability. Interested friends can take a look.

To quickly submit your solution, use UVa Quick Submit .

Your UVa username :

Search Problem Number :   Clear

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View Live Submissions

What is uHunt

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PHP Bonus - Online Judge Problem Solving Using PHP

• Problem Solving
• Popular online problem solving platforms -
• How we’ll solve the problems ?
• Test in Online PHP Code -
• Problem Details
• Solution in PHP
• Problem 2: Extremely Basic
• Problem 3 - Multiples
• Problem 4 - Upto Odd Numbers

¶ Problem Solving

Todays, lecture is totally different from PHP programming language aspects. This is for both PHP and all general level programming languages. Problem Solving is a very important parts to becoming a very good Developer. In our practical life, we’ll face lots of different and new problems every day when we starts our real project. If we have proper problem solving skills, then we can easily solve any problem easily. Let’s try solving some of the problems in popular online judge programming platform.

¶ Popular online problem solving platforms -

• BeeCrowd - https://www.beecrowd.com.br/judge/en
• UVA Online Judge - https://onlinejudge.org
• HackerRank Online Judge - https://www.hackerrank.com
• CodeForce Online Judge - https://codeforces.com
• LightOJ - https://lightoj.com

And there is so many online programming problem solving websites out there. We’ll start using the easiest one BeeCrowd or previously URI online judge.

¶ BeeCrowd Online Judge

BeeCrowd is one of the most popular online judge problem solving platform. It’s suitable for both beginner, intermediate and advanced level users.

¶ How we’ll solve the problems ?

We’ll read the problems very carefully and solve those problems using PHP language. You can try with other language also if you’re comfortable. We’ll solve some Beginner Level problems - https://www.beecrowd.com.br/judge/en/problems/index/1

¶ Test in Online PHP Code -

https://onecompiler.com/php/3y5ak79p9

¶ Problem 1: Hello World

https://www.beecrowd.com.br/judge/en/problems/view/1000

¶ Problem Details

Hello world.

Welcome to beecrowd !

Your first program in any programming language is usually " Hello World! ". In this first problem all you have to do is print this message on the screen.

This problem has no input.

You must print the message Hello World! and then the endline as shown below.

¶ Solution in PHP

¶ problem 2: extremely basic.

https://www.beecrowd.com.br/judge/en/problems/view/1001

¶ Problem 3 - Multiples

https://www.beecrowd.com.br/judge/en/problems/view/1044

¶ Problem 4 - Upto Odd Numbers

https://www.beecrowd.com.br/judge/en/problems/view/1067

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Tutorial for problem-setters

This is tutorial for those who want to add new problems to SPOJ problemsets. It will get you through a little complex procedure of adding the problems.

Who is a problem-setter?

How to become a problem-setter, how to begin, problem page, adding a problem, adding non-english problem, editing an existing problem, why my problem is not visible for other users (editorial board issue), communicating with users solving your problem.

It's a person who adds new problems.

If you are interested in helping us build our problem base, or if you would like to organize a contest of your own, please fill out the form (if you are a newcomer to the site, or have solved fewer than 20 problems, a little additional information about yourself is welcome but by no means obligatory). You will receive a reply with detailed instructions. Please note that we reserve the right to correct, recategorise, modify or remove problems which are available for solving in one of main problemsets. This does not concern problems added specifically for use in private contests which are not visible in the archives of the online judge.

You can start here ( problem list → My own problems ) There you can view all problems you have already added. To create a new problem click on Add problem link at the top of the page.

As mentioned before, to add a new problem click (of course first you should log in) on problem list link on the left side of the page and then click the Add problem link at the top of the page. To edit existing problem go to its page and click on the Edit problem link at the top of the page.

Here is the screen shot with some sample data:

Meaning of the fields (when you add or edit a problem, move pointer over green question marks to see the hints):

Problem code

Unique problem code, consists of 3 up to 8 capital letters.

Problem name

Just a problem name that will be displayed in the NAME column in the problem list.

Group of people related to this problem. The group must exist (you can create new one clicking "add group" menu position). You should use this option if you want to grant access to your problems for the other co-authors.

Source code limit

Source code size limit of solutions sent by users, measured in bytes.

Origin of the problem (for example link).

Assessment type

Type of the problem:

• Binary - "accepted" or "wrong answer".
• Maximize score - higher score means better solution.
• Minimize score - lower score means better solution.

How the problem is judged:

• Pyramid (PIII733) - automatically.
• Manual - manually by problem setter.

Placement in main problem set

Choose in which section your problem should appear. If you are going to test your problem, it's incomplete or has no English version, check none option - problem will be available only for you.

Check this option if you want to allow submitting solutions to this problem.

Available for use in 3rd party contests

Is it possible to use your problem in other contests?

Dynamic content

This option is deprecated.

Master judge

Source code of high level judge - used for multiple test cases and for single data set as well. You can write your own judge, if you need it. Default judge ( 1000 . Generic masterjudge) accepts solution only if all test cases are accepted. If you want the score to be percentage of accepted test cases, choose judge no. 1001 .

Available languages

Mark languages in which users can upload solutions.

Include new languages

If this option is enabled, all languages added in the future will be automatically available for this problem.

Merge all languages in ranklist

If this option is disabled participant will appear in problem's score table as many times as he changes programming language. In other case he'll appear only once.

Continue editing after saving changes

If you check this option and click Modify , problem description (modified version) will appear in new window.

Create the problem.

Check how the problem page will look like.

After you fill all required fields, click Create button and the problem will be created. Then go back to problem's page, click on the Edit problem link and continue setting problem with new options now available.

First, you have to set Placement in main problemset to none so that your problem won't be visible in the main problemset (i.e. in the main contest that is accessible to everyone and can be viewed right after entering www.spoj.com ). This is enough for you to use your problem in your own private contest (how to set up your own contest? - answers can be found in this tutorial ). If you want to allow other problem setters to use your problem in their contests, check the Available for use in 3rd party contests option. On the other hand, if you want your problem to appear in some contest (either private or public - www.spoj.com , br.spoj.com , pl.spoj.com , vn.spoj.com ...), contact with the contest setter.

Example screen shot: (with DOUGHNUT problem data).

Meaning of the fields:

Problem test sequence

Define here which test cases use to judge solution (you can upload test cases below in

Test data upload section).

Test data upload

Here you can add, modify or delete test cases and choose judges for them. Usually one

data set suffices but if you wish you can add more.

Add/modify testcase

Choose whether you want to add new test case or to modify or delete existing one. Notice: Input and output text files are uploaded from the specified file on your computer. The provided file may be zipped or gzipped; if so, make sure it has the correct extension (.gz and .zip respectively). The size of the uploaded file may not exceed 1MB.

Choose input data (if the problem has no input data you can leave this field empty).

Output file

Choose output file (if the problem has many correct answers you can leave this field empty

and use your own judge).

Test case judge

Add your own judge or choose between predetermined judges:

Ignores extra whitespaces - the standard judge for binary problems, i.e. problems of ACM-type: Accepted or Wrong Answer. Output generated by submitted program should be the same as test case output file (except whitespaces).

Ignores FP rounding UP tp 10 -2 - the standard judge for binary problems, i.e. problems of ACM-type: Accepted or Wrong Answer. Use this judge for numerical problems. Output generated by submitted program should be the same as test case output (with accuracy 0.01).

Ignores FP rounding UP tp 10 -6 - the standard judge for binary problems, i.e. problems of ACM-type: Accepted or Wrong Answer. Use this judge for numerical problems. Output generated by submitted program should be exactly the same as test case output (with accuracy 0.000001).

Score is source length - score for the solution equals to length of source code (only if test case was accepted).

Rejudge submission range

All submissions within specified range will be rejudged. Use this feature only in case of invalid data set, your own invalid judge, etc.

Remember : rejudging many solutions may take a lot of time - better check your test cases twice and try to avoid rejudging solutions.

In case of any problems with data sets, wrong problem statements, etc., Editorial Board Members are allowed to hide the problem; after that the problem-setter will see the problem for his own purposes only - the problem will be invisible for others, and only EB Members can show it again.

To communicate with users post comments under the problem. You can use comments to publish information about test cases changes, reason of the re-judgment, etc. Please also check for new comments from users from time to time and answer their questions - comments are the main way of communication between users and problem-setters.

For more information, help, hints, ideas and suggestions please visit our forum . FAQ is also situated in the forum .

For now there is no comprehensive tutorial about judges but it will appear soon. You can find some useful information about writing judges here: https://www.spoj.com/forum/viewtopic.php?t=3388

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Climate advocates want to solve their 'biggest problem' in the US: Turning out voters

ABC News embedded with an environmental group canvassing in Pittsburgh.

ABC News is taking a look at solutions for issues related to climate change and the environment with the series "The Power of Us: People, The Climate, and Our Future."

In battleground states across the country, environmental activists like Dr. Emily Church are canvassing on behalf of an organization called the Environmental Voter Project in an effort to turn out people who care the most about climate change -- but who haven't shown up for past elections.

During a recent effort in Pittsburgh, Church, a biology professor who leads local canvasses for the project, recalled to ABC News how she used to lobby lawmakers directly to take action on climate change, but they told her voters don't care about the issue.

She said she's now trying to prove them wrong.

"The people who prioritize climate and the environment need to show up," Church said. "That's how we're going to get anything done."

The Environmental Voter Project, or EVP, is targeting very specific individuals: registered voters who list climate change as their No. 1 issue but who are unlikely to cast ballots in November's election based on their voting history.

"Our biggest problem in the climate movement right now [is] we don't have enough voting power," EVP founder and executive Nathaniel Stinnett said.

EVP takes a targeted approach to door knocking, Stinnett explained. Using polling, the group first determines which registered voters in a particular area, like Pittsburgh, would rank climate as their top voting issue. They then cross-reference profiles with voting records to find people who have not come out to the polls recently or regularly.

By Stinnett's accounting, the group has been successful across general elections, primaries and even in local races.

"We've sometimes increased turnout by as much as 1.8 percentage points in general elections, 3.6 points in primaries and 5.7 points in local elections," he said, noting that while 1.8 percentage points might sound small, it could determine an election. Pennsylvania, for example, was only won by President Joe Biden in 2020 by 1.17%.

For the canvassing effort in Pittsburgh, Stinnett said EVP targeted people who didn't vote in the 2020 election or elections in the years since. He added that they identified 22,135 voters in the city who are highly likely to rank climate as their top priority but unlikely to vote in November.

The group claims nonpartisanship but acknowledges that right now it's Democrats working on climate change almost exclusively. One of their hopes is to bring more Republicans to the table, too.

"We want to scare the bejesus out of as many politicians as possible, no matter what side of the aisle they're on, until they think, 'You know what, the only way I can win elections is if I start recognizing the biggest crisis,'" Stinnett said.

MORE:Coastal US cities are sinking as sea levels continue to rise, new research shows

Over time, climate change has become a more salient voting issue. In 2010, only a slim majority of Americans agreed that global warming was occurring, according to polling by the Yale Program on Climate Change Communication . Now, 72% of Americans agree.

But climate is currently not one of the biggest motivators for people this election cycle, surveys have indicated -- though climate advocates hope to change the electorate by encouraging turnout of climate-concerned voters.

According to a February poll by the Wall Street Journal , registered voters listed immigration (20%), the economy (14%), abortion (8%) and democracy (8%) as their top issues. Climate change ranked 11th, with 2% of voters choosing it as their top issue.

More broadly, Gallup's tracking of what Americans say is the country's most important problem over time shows climate, pollution and the environment at 2% in March, far below economic issues and immigration.

Polling has also shown that in addition to a partisan divide on the issue, a generational shift may be at play.

"Young voters in general tend to be more Democratic, and that is kind of tied up inextricably with their belief that climate is really important," said Nathaniel Rakich, a senior editor and senior elections analyst at 538. "So if Republicans don't want to basically be losing this upcoming electorate by large margins for decades to come, they're going to have to eat into that Democratic support by at least proposing some solutions and addressing climate change."

Even the Biden administration, which has prioritized fighting climate change, is being pushed by progressives to do more on the issue.

Twenty-one activists with the environmental advocacy group Sunrise Movement were arrested outside of Biden campaign headquarters in Wilmington, Delaware, in February. That group and other advocates have additional demonstrations planned in the run-up to the November election.

"I think there were some missteps by the administration -- permitting the Willow project in Alaska was a step backwards. That was unfortunate," Evergreen Action Executive Director Lena Moffitt said, referring to a large-scale oil drilling initiative backed by Alaska lawmakers and others in the state for its economic value, but which environmentalists criticized as undercutting the White House's climate goals.

MORE: Biden admin approves Willow Project despite oil drilling concerns

"We know that we need to move away from fossil fuels and, at the same time, the administration is doing a lot to hasten that move away from fossil fuels," Moffitt said.

The choice for voters in November, on the issue of climate, is stark. President Biden has spoken urgently of the dangers of not slowing climate change and has pushed renewable energy solutions, backed electric vehicle infrastructure and created a new Climate Corps to train and expand the environmental workforce.

Biden last week finalized new protections against oil and gas production for some13 million acres of land in Alaska and, through the Environmental Protection Agency, has imposed aggressive emissions standards for vehicles to cut future greenhouse gases.

Meanwhile, former President Donald Trump, who has long questioned climate science, without evidence, has opposed Biden's clean energy policies and promised to roll them back -- arguing they are a drag on the economy and make the U.S. less competitive and independent.

MORE: EPA sets new emissions standards for heavy-duty vehicles in effort to fight climate change

"The fact is President Biden has done more to address climate change than any president in U.S. history. And there's a lot more to be done," Moffitt said. "Scientists have said that we still can avoid the worst of the worst of the climate crisis. But what we do in these next few years is essential to which path we choose."

Stinnett agreed, telling ABC News that too often Americans have been told to focus on their own individual habits rather than government policy.

"[Politicians say,] 'Hey, don't pay attention to that coal-fired power plant back there. Instead, it's all your fault for having a plastic water bottle in your hand.' And we bought it. We bought it hook, line and sinker," he said. "In truth, it is far more of a political and a systemic problem that needs political and systemic solutions."

In Pittsburgh, Church said that despite the difficulty in getting new, environmentally minded voters to the polls, she thinks the challenge is worth it.

"The science is very clear. So we know what we need to do," she said, "it's just a matter of getting it done."

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24/7 coverage of breaking news and live events

3 Ways to Strengthen Math Instruction

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Students’ math scores have plummeted, national assessments show , and educators are working hard to turn math outcomes around.

But it’s a challenge, made harder by factors like math anxiety , students’ feelings of deep ambivalence about how math is taught, and learning gaps that were exacerbated by the pandemic’s disruption of schools.

This week, three educators offered solutions on how districts can turn around poor math scores in a conversation moderated by Peter DeWitt, an opinion blogger for Education Week.

Here are three takeaways from the discussion. For more, watch the recording on demand .

1. Intervention is key

Research shows that early math skills are a key predictor of later academic success.

“Children who know more do better, and math is cumulative—so if you don’t grasp some of the earlier concepts, math gets increasingly harder,” said Nancy Jordan, a professor of education at the University of Delaware.

For example, many students struggle with the concept of fractions, she said. Her research has found that by 6th grade, some students still don’t really understand what a fraction is, which makes it harder for them to master more advanced concepts, like adding or subtracting fractions with unlike denominators.

At that point, though, teachers don’t always have the time in class to re-teach those basic or fundamental concepts, she said, which is why targeted intervention is so important.

Still, Jordan’s research revealed that in some middle schools, intervention time is not a priority: “If there’s an assembly, or if there is a special event or whatever, it takes place during intervention time,” she said. “Or ... the children might sit on computers, and they’re not getting any really explicit instruction.”

2. ‘Gamify’ math class

Students today need new modes of instruction that meet them where they are, said Gerilyn Williams, a math teacher at Pinelands Regional Junior High School in Little Egg Harbor Township, N.J.

“Most of them learn through things like TikTok or YouTube videos,” she said. “They like to play games, they like to interact. So how can I bring those same attributes into my lesson?”

Part of her solution is gamifying instruction. Williams avoids worksheets. Instead, she provides opportunities for students to practice skills that incorporate elements of game design.

That includes digital tools, which provide students with the instant feedback they crave, she said.

But not all the games are digital. Williams’ students sometimes play “trashketball,” a game in which they work in teams to answer math questions. If they get the question right, they can crumble the piece of paper and throw it into a trash can from across the room.

“The kids love this,” she said.

Williams also incorporates game-based vocabulary into her instruction, drawing on terms from video games.

For example, “instead of calling them quizzes and tests, I call them boss battles,” she said. “It’s less frightening. It reduces that math anxiety, and it makes them more engaging.

“We normalize things like failure, because when they play video games, think about what they’re doing,” Williams continued. “They fail—they try again and again and again and again until they achieve success.”

3. Strengthen teacher expertise

To turn around math outcomes, districts need to invest in teacher professional development and curriculum support, said Chaunté Garrett, the CEO of ELLE Education, which partners with schools and districts to support student learning.

“You’re not going to be able to replace the value of a well-supported and well-equipped mathematics teacher,” she said. “We also want to make sure that that teacher has a math curriculum that’s grounded in the standards and conceptually based.”

Students will develop more critical thinking skills and better understand math concepts if teachers are able to relate instruction to real life, Garrett said—so that “kids have relationships that they can pull on, and math has some type of meaning and context to them outside of just numbers and procedures.”

It’s important for math curriculum to be both culturally responsive and relevant, she added. And teachers might need training on how to offer opportunities for students to analyze and solve real-world problems.

“So often, [in math problems], we want to go back to soccer and basketball and all of those things that we lived through, and it’s not that [current students] don’t enjoy those, but our students live social media—they literally live it,” Garrett said. “Those are the things that have to live out in classrooms right now, and if we’re not doing those things, we are doing a disservice.”

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