Your Java Toolbox for Challenges

Reading input with Scanner

Every challenge gives you input as text. Most of the time it starts with a count n, then n values. Scanner reads them for you, ignoring spaces and new lines:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] marks = new int[n];
        for (int i = 0; i < n; i++) marks[i] = sc.nextInt();
        System.out.println("I read " + n + " marks, the last one is " + marks[n - 1]);
    }
}

Input (what the user types)

4
70 85 92 64

Output

I read 4 marks, the last one is 64

Common mistake: Never write while (sc.hasNext()) in a challenge. The browser never says “the input is over”, so the loop never ends. Read exactly n items instead.

int or long?

An int stops at about 2.1 billion. If a number goes past that, it silently wraps around to a wrong, often negative, value. A long goes up to about 9 × 10¹⁸:

public class Main {
    public static void main(String[] args) {
        int big = 2000000000;
        int wrong = big + big;           // too big for int!
        long right = (long) big + big;   // turn one into long first
        System.out.println(wrong);
        System.out.println(right);
    }
}

Output

-294967296
4000000000

Printing arrays and lots of output

System.out.println(arr) prints something useless like [I@6d06d69c. Use Arrays.toString(arr) while testing. For the real answer, build the text with a StringBuilder and print it once – it's much faster than many small prints:

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] squares = new int[6];
        for (int i = 0; i < squares.length; i++) squares[i] = i * i;

        System.out.println(Arrays.toString(squares));   // good for checking

        StringBuilder sb = new StringBuilder();         // good for answers
        for (int i = 0; i < squares.length; i++) {
            if (i > 0) sb.append(' ');
            sb.append(squares[i]);
        }
        System.out.println(sb);
    }
}

Output

[0, 1, 4, 9, 16, 25]
0 1 4 9 16 25

Remember: Read with Scanner, use long for big totals, and print answers exactly in the format the challenge asks for – one extra space or word makes the check fail.