A solution to Beautiful Days (hackerrank) in Java 8

Solution.java

import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;

public class Solution {

    // Complete the beautifulDays function below.
    static int beautifulDays(int i, int j, int k) {
        int numberOfBeautifulDays = 0;
        for (int day = i; day <= j; day++) {
            if (isBeautiful(day, k)) numberOfBeautifulDays++;
        }
        return numberOfBeautifulDays;
    }

    static boolean isBeautiful(int day, int divisor) {
        int reverseDay = getReverseNumber(day);
        if (Math.abs((day - reverseDay)%divisor) == 0) return true;
        return false;
    }

    static int getReverseNumber(int number) {
        StringBuilder reverseString = new StringBuilder();
        reverseString.append(number);
        reverseString = reverseString.reverse();
        int reverseNumber = Integer.parseInt(reverseString.toString());
        return reverseNumber;
    }

    /**
     * This is the first version I implemented before knowing that Stringbuilder
     * could be used. Here for information only.
     *
     * @param number to reverse (example 123456)
     * @return a number reverted (example 654321)
     */
    static int getReverseNumber_alternative(int number) {
        int remainingNumber = number;
        String reverseNumberString = "";
        while (remainingNumber >= 10) {
            int lastDigit = remainingNumber % 10;
            remainingNumber = remainingNumber / 10;
            reverseNumberString += String.valueOf(lastDigit);
        }
        reverseNumberString += String.valueOf(remainingNumber);
        return Integer.parseInt(reverseNumberString);
    }
    
    private static final Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) throws IOException {
        BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

        String[] ijk = scanner.nextLine().split(" ");

        int i = Integer.parseInt(ijk[0]);

        int j = Integer.parseInt(ijk[1]);

        int k = Integer.parseInt(ijk[2]);

        int result = beautifulDays(i, j, k);

        bufferedWriter.write(String.valueOf(result));
        bufferedWriter.newLine();

        bufferedWriter.close();

        scanner.close();
    }
}

Actually my first version doesn't work with some test cases. I'll have to investigate further.

The problem is at this function:

    static int getReverseNumber_wrong(int number) {
        int remainingNumber = number;
        String reverseNumberString = "";
        while (remainingNumber > 10) {
            int lastDigit = remainingNumber % 10;
            remainingNumber = remainingNumber / 10;
            reverseNumberString += String.valueOf(lastDigit);
        }
        reverseNumberString += String.valueOf(remainingNumber);
        return Integer.parseInt(reverseNumberString);
    }

The solution (and also a better implementation) is to use StringBuilder to reverse the number, like so:

    static int getReverseNumber(int number) {
        StringBuilder reverseString = new StringBuilder();
        reverseString.append(number);
        reverseString = reverseString.reverse();
        int reverseNumber = Integer.parseInt(reverseString.toString());
        return reverseNumber;
    }

But I'm invetigating my previous function to understand what was wrong.

Oh, crap! Hahahaha.

I compared the results of both functions and they only differ when the number starts with 10, so the problem was this line

while (remainingNumber > 10) {

which should be:

while (remainingNumber >= 10) {