Find Strong Numbers in Range()

Step 1

The first step is defining the helper function, factorial, which will find the factorial of a given number. Factorial of a number is the product of all positive integers less than or equal to that number.

public static int factorial(int num) {
    int fact = 1;
    for(int i = 1; i <= num; i++) {
        fact = fact * i;
    }
    return fact;
}

Step 2

Next, we define another helper function called isStrongNumber. This function will determine if a given number is a strong number. A strong number is a number in which the sum of the factorial of its digits is equal to the number itself.

public static boolean isStrongNumber(int num) {
    int temp = num, sum = 0;
    while(temp != 0) {
        int remainder = temp % 10;
        sum = sum + factorial(remainder);
        temp = temp / 10;
    }
    return sum == num;
}

Step 3

Now, we need to build the main function findStrongNumbersInRange which will find all the strong numbers within a given range. We traverse from the start to the end of the range and for each number, we check if it is a strong number using the isStrongNumber function. If it is, we add it to the array of strong numbers.

public static ArrayList<Integer> findStrongNumbersInRange(int start, int end) {
    ArrayList<Integer> strongNumbers = new ArrayList<>();
    for(int i = start; i <= end; i++) {
        if(isStrongNumber(i)) {
            strongNumbers.add(i);
        }
    }
    return strongNumbers;
}

Step 4

The final step would be to write a main function to test the above code. Here we will simply call the findStrongNumbersInRange function with a test range and print out the results.

public static void main(String[] args) {
    System.out.println(findStrongNumbersInRange(1, 500));
}

Conclusion

This approach uses a helper function to calculate the factorial of numbers, and another helper function to determine if a number is a strong number. Then a main function iterates over the given range and finds all the strong numbers within it. Here's the full implementation:

public class StrongNumber {
  public static int factorial(int num) {
    int fact = 1;
    for(int i = 1; i <= num; i++) 
        fact = fact * i;
    return fact;
  }

  public static boolean isStrongNumber(int num) {
    int temp = num, sum = 0;
    while(temp != 0) {
        int remainder = temp % 10;
        sum = sum + factorial(remainder);
        temp = temp / 10;
    }
    return sum == num;
  }

  public static ArrayList<Integer> findStrongNumbersInRange(int start, int end) {
    ArrayList<Integer> strongNumbers = new ArrayList<>();
    for(int i = start; i <= end; i++) {
        if(isStrongNumber(i)) 
            strongNumbers.add(i);
    }
    return strongNumbers;
  }

  public static void main(String[] args) {
    System.out.println(findStrongNumbersInRange(1, 500));
  }
}

So, following this approach, you can find all the strong numbers within a given range. You can also modify this code to suite your specific needs.