Java/src/main/java/com/thealgorithms/maths/KaprekarNumbers.java

package com.thealgorithms.maths;

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;

public final class KaprekarNumbers {
    private KaprekarNumbers() {
    }

    /* This program demonstrates if a given number is Kaprekar Number or not.
        Kaprekar Number: A Kaprekar number is an n-digit number which its square can be split into
       two parts where the right part has n digits and sum of these parts is equal to the original
       number. */

    // Provides a list of kaprekarNumber in a range
    public static List<Long> kaprekarNumberInRange(long start, long end) throws Exception {
        long n = end - start;
        if (n < 0) {
            throw new Exception("Invalid range");
        }
        ArrayList<Long> list = new ArrayList<>();

        for (long i = start; i <= end; i++) {
            if (isKaprekarNumber(i)) {
                list.add(i);
            }
        }

        return list;
    }

    // Checks whether a given number is Kaprekar Number or not
    public static boolean isKaprekarNumber(long num) {
        String number = Long.toString(num);
        BigInteger originalNumber = BigInteger.valueOf(num);
        BigInteger numberSquared = originalNumber.multiply(originalNumber);
        if (number.length() == numberSquared.toString().length()) {
            return number.equals(numberSquared.toString());
        } else {
            BigInteger leftDigits1 = BigInteger.ZERO;
            BigInteger leftDigits2;
            if (numberSquared.toString().contains("0")) {
                leftDigits1 = new BigInteger(numberSquared.toString().substring(0, numberSquared.toString().indexOf("0")));
            }
            leftDigits2 = new BigInteger(numberSquared.toString().substring(0, (numberSquared.toString().length() - number.length())));
            BigInteger rightDigits = new BigInteger(numberSquared.toString().substring(numberSquared.toString().length() - number.length()));
            String x = leftDigits1.add(rightDigits).toString();
            String y = leftDigits2.add(rightDigits).toString();
            return (number.equals(x)) || (number.equals(y));
        }
    }
}