JavaAlgorithms/Maths/VampireNumber.java

package Maths;

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

/**
 * 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>
 */
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();
  }
}