Problem: Write a Java program to check whether a given number is a Fascinating number or not.

A Fascinating number is the number which when combined with its multiple of 2 and 3 results into a number that consists of all digits from 1 to 9 exactly once.

Example: 192

Fascinating number

Steps to Check Fascinating number in Java:

  1. Input a number.
  2. Compute its multiple of 2 and 3.
  3. Concatenate the multiple calculated in the previous step with the input number.
  4. Count the frequency of each digit in the concatenated number.
  5. Check if all the counts are exactly 1.
  6. If so, then the input number is a fascinating number otherwise not.

Here is the Java program, which implements the above step to verify whether the input number is fascinating number or not:

import java.util.Scanner;

public class Main
{
  public static void main(String[] args) {
    Scanner in=new Scanner(System.in);
	    
    //1. Input a number
    System.out.println("Enter a number");
		int num=in.nextInt();
		
    //2. Compute its multiple of 2 and 3.
    int mulBy2 = num*2;
    int mulBy3 = num*3;
    
    //3. Cancatenate numbers
    int newNumber = 0;
    newNumber += num;
    newNumber = newNumber * (int)Math.pow(10,countDigits(mulBy2))	+ mulBy2;
    newNumber = newNumber * (int)Math.pow(10,countDigits(mulBy3))	+ mulBy3;
        
    //4. Count the frequency of each digit
    int count[] = new int[10];  
    while(newNumber>0){
      ++count[newNumber%10];
      newNumber = newNumber/10;
    }
        
    //5. checking if all counts are 1.
    boolean flag = true; 
    for(int i=1; i<10; i++){
      /*
        *If any digit appears more than one in the concatenated number-
        * then it is not a fascinating number
      */
      if(count[i] != 1){
        flag = false;
        break;
      }
    }
    
    //6. Print the result
    if(flag)
      System.out.println("Fascinating Number");
    else
      System.out.println("Non Fascinating Number");
            
  }
  
  //Function to return count of digits in a Number
  private static int countDigits(int num){
    int count=0;
    while(num>0){
      count++;
      num=num/10;
    }
    return count;
  }
}

Output:

Enter a number
192
Fascinating Number

Explanation:

To concatenate the multiples with the input number, we use the count of digits as follows:

//multiple of 2
192*2 = 384 (3 digits)
192*(10^3) = 192000
192000+384 = 192384

//multiple of 3
192*3 = 576 (3 digits)
192384*(10^3) = 192384000
192384000+576 = 192384576

We store the count of each digit in an array and then examine the array whether all the counts are 1 or not.

We then print the result depending on the result whether the above conditions were satisfied or not.

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