TUTORIAL.md

Tutorial

The purpose of this document is to help you get started on solving the challenges.

If you're used to competitive programming, you don't need to read this tutorial as you probably won't learn anything new.

Table of contents

• Input/Output
• How to run your programs
• Testing your solutions

Input/Output

Unless specifically stated in the challenge description, your program should always read from stdin and write to stdout. How to do that depends on the language you are using.

Here are some examples.

I/O with Python

from sys import stdin, stdout


for line in stdin:
    # Do someting with data, then output the result.
    stdout.write(answer)

I/O with C++

#include <iostream>
#include <string>

using namespace std;

int main(void)
{
    string line;

    while (getline(cin, line)) {
        // Solve test case, then output the result.
        cout << answer << endl;
    }

    return 0;
}

I/O with NodeJS

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.on('line', (line) => {
  // Do something with the data and output the result.
  console.log(answer);
});

I/O with Java

import java.util.Scanner;

public class Solution {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        // Scanner can be really slow on large inputs.
        // Use a buffered reader when dealing with large inputs.
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();

            // Solve the test case and output its answer.
            System.out.println(answer);
        }
    }
}

How to run your programs

Once you are done writting your solution, you can compile it and then run it on test data with I/O redirections.

Python

python program.py < in.txt > out.txt

C++

g++ -o program -std=c++17 main.cpp
./program < in.txt > out.txt

NodeJS

node program.js < in.txt > out.txt

Java

javac MyProgram.java
java MyProgram < in.txt > out.txt

You can then check your program results by inspecting the out.txt file.

Testing your solutions

Each challenge comes with official tests data you should run your solution against to validate it. There are usually two files:

• in.official.txt which contains official input data.
• out.official.txt which contains the output expected from your solution.

A typical workflow will be:

1. Run your solution with the official input.

   python solution.py < in.official.txt > out.txt

2. Check if your solution is correct by comparing its output to the official answer.

   diff out.txt out.official.txt

   If the two files are identical, diff will exit with no message. Otherwise, it will show you the lines where the two files differ.

IMPORTANT

Your program can give correct answers for the sample testcases, but fails on the official ones. Keep in mind that the official testcases are trickier than the samples. Failing on official tests mean that your solution needs to be improved.