JavaAlgorithms/Maths/VampireNumber.java

package Maths;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 n number theory, a vampire number (or true vampire number) is a composite natural number with an even number of digits,
 that can be factored into two natural numbers each with half as many digits as the original number
 and not both with trailing zeroes, where the two factors contain precisely
 all the digits of the original number, in any order, counting multiplicity.
 The first vampire number is 1260 = 21 × 60.
 * *
 * <p>
 * * link: https://en.wikipedia.org/wiki/Vampire_number
 * * </p>
 * <p>
 *
 */







public class VampireNumber {

    public static void main(String[] args) {

      test(10,1000);
    }

     static void  test(int  startValue,  int  stopValue) {
         int countofRes = 1;
         StringBuilder res = new StringBuilder();


         for (int i = startValue; i <= stopValue; i++) {
             for (int j = i; j <= stopValue; j++) {
                 // System.out.println(i+ " "+ j);
                 if (isVampireNumber(i, j,true)) {
                     countofRes++;
                     res.append("" + countofRes + ": = ( " + i + "," + j + " = " + i*j + ")" + "\n");
                 }
             }
         }
         System.out.println(res);
     }




        static boolean isVampireNumber(int a, int b, boolean noPseudoVamireNumbers ) {

        // this is for pseudoVampireNumbers  pseudovampire number need not be of length n/2 digits for example
            // 126 = 6 x 21
            if (noPseudoVamireNumbers) {
                if (a * 10 <= b || b * 10 <= a) {
                    return false;
                }
            }

            String mulDigits = splitIntoDigits(a*b,0);
             String faktorDigits = splitIntoDigits(a,b);

            return mulDigits.equals(faktorDigits);
            }



// methode to Split the numbers to Digits
        static String splitIntoDigits(int num, int num2) {

        StringBuilder res = new StringBuilder();

            ArrayList<Integer> digits = new ArrayList<>();
            while (num > 0) {
                digits.add(num%10);
                num /= 10;
            }
            while (num2 > 0) {
                digits.add(num2%10);
                num2/= 10;
            }
            Collections.sort(digits);
            for (int i : digits) {
                res.append(i);
            }


            return res.toString();
        }
}