Here is my c++ solution, you can watch the explanation here : https://youtu.be/snADwNvEEcM

vector<int> countingSort(vector<int> arr) {
    vector<int> count(100, 0);
    for(int i = 0; i < arr.size(); i++) count[arr[i]]++;
    return count;
}

Java

public static List<Integer> countingSort(List<Integer> arr) {

    List<Integer> list=new ArrayList<>();
    for(int i=0;i<=99;i++){
        list.add(0);
    }
    for(int i=0;i<arr.size();i++){
         list.set(arr.get(i), list.get(arr.get(i))+1);   
    }  
    return list;
}

import java.io.; import java.math.; import java.security.; import java.text.; import java.util.; import java.util.concurrent.; import java.util.function.; import java.util.regex.; import java.util.stream.*; import static java.util.stream.Collectors.joining; import static java.util.stream.Collectors.toList;

class Result {

/*
 * Complete the 'countingSort' function below.
 *
 * The function is expected to return an INTEGER_ARRAY.
 * The function accepts INTEGER_ARRAY arr as parameter.
 */

public static List<Integer> countingSort(List<Integer> arr) {

 Integer countArr[] = new Integer[100];
 Arrays.fill(countArr, 0);
 for(int i=0;i<arr.size();i++){
     countArr[arr.get(i)]++;         
 }  
  return Arrays.asList(countArr);  


}

}

public class Solution { public static void main(String[] args) throws IOException { BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in)); BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));

    int n = Integer.parseInt(bufferedReader.readLine().trim());

    List<Integer> arr = Stream.of(bufferedReader.readLine().replaceAll("\\s+$", "").split(" "))
        .map(Integer::parseInt)
        .collect(toList());


    List<Integer> result = Result.countingSort(arr);

    bufferedWriter.write(
        result.stream()
            .map(Object::toString)
            .collect(joining(" "))
        + "\n"
    );

    bufferedReader.close();
    bufferedWriter.close();
}

}