Digital Sum Sort

Sort sum digits from elements in a collection.


You are given an array of integers. Sort it in such a way that if A comes before B then the sum of digits of A is less than or equal to the sum of digits of B.



collection = [15, 20, 4, 8]

The output should be:

digitalSumSort(collection) = [2, 4, 6, 8]


import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.stream.Collectors;

public class DigitalSumSorter {

  private List<Integer> sort(List<Integer> numbers){
    List<String> numbersAsString = numbers.stream().map(it -> it.toString()).collect(Collectors.toList());
    List<Integer> result = numbersAsString.stream().map(it -> it.chars().map( ch -> Integer.parseInt(Character.toString((char) ch))).sum()).sorted().collect(Collectors.toList());
    return result;

  public static void main(String[] args){
    List<Integer> numbers = Arrays.asList(15, 20, 4, 8);
    List<Integer> result = new DigitalSumSorter().sort(numbers);
    assert Arrays.asList(2,4,6,8).equals(result);


To download the code:

git clone https://github.com/josdem/algorithms-workshop.git
cd digital-sum-sort

To run the code:

javac DigitalSumSorter.java
java -ea DigitalSumSorter

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