How to Write Number in Expanded Form in Java

The challenge

You will be given a number and you will need to return it as a string in Expanded Form.

Examples:

Challenge.expandedForm(12); // Should return "10 + 2"
Challenge.expandedForm(42); // Should return "40 + 2"
Challenge.expandedForm(70304); // Should return "70000 + 300 + 4"

NOTE: All numbers will be whole numbers greater than 0.

The solution in Java code

Option 1:

public class Challenge {
    public static String expandedForm(int num) {
      StringBuffer res = new StringBuffer();
      int d = 1;
      while(num > 0) {
        int nextDigit = num % 10;        
        num /= 10;
        if (nextDigit > 0) {
          res.insert(0, d * nextDigit);
          res.insert(0, " + ");
        }
        d *= 10;
      }
      return res.substring(3).toString();
    }
}

Option 2:

import java.util.LinkedList;

public class Challenge {
    public static String expandedForm(int num) {
        LinkedList<String> expandedList = new LinkedList<>();
        int digit;
        int multiplier = 1;
        while (num > 0) {
            digit = (num % 10) * multiplier;
            if (digit != 0) expandedList.push(Integer.toString(digit));
            num /= 10;
            multiplier *= 10;
        }
        return String.join(" + ", expandedList);
    }
}

Option 3:

import java.util.stream.IntStream;
import static java.util.stream.Collectors.joining;

public class Challenge {
  public static String expandedForm(int num) {
    var ns = ""+num;
    return IntStream.range(0, ns.length())
      .mapToObj(i -> ns.charAt(i) + "0".repeat(ns.length() - 1 - i))
      .filter(e -> !e.matches("0+"))
      .collect(joining(" + "));
  }
}

Test cases to validate our solution

import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;

public class SolutionTest {
    @Test
    public void testSomething() {
        assertEquals("10 + 2", Challenge.expandedForm(12)); 
        assertEquals("40 + 2", Challenge.expandedForm(42)); 
        assertEquals("70000 + 300 + 4", Challenge.expandedForm(70304)); 
     }
}