How to arrange an array in decreasing order of frequency of each number?

Question

Input : {5, 13, 6, 5, 13, 7, 8, 6, 5}

Output : {5, 5, 5, 13, 13, 6, 6, 7, 8}

The question is to arrange the numbers in the array in

Answer 1

private static void sortByFrequency(int[] a)
{
    Map map = new HashMap();
    for(int i=0; i set = map.keySet();
    TreeSet treeSet = new TreeSet();
    for(int i : set)
    {
        treeSet.add(map.get(i));
    }

    for(Element e : treeSet)
    {
        for(int i=0; i
{
    private final int index;
    private int frequency;

    Element(int index)
    {
        this.index = index;
        this.frequency = 1;
    }

    @Override
    public int compareTo(Element o)
    {
        int k = o.frequency - this.frequency;
        if(k != 0) return k;
        else
        {
            return this.index - o.index;
        }
    }
}

public static void main(String[] args)
{
    int[] a = {5, 13, 6, 5, 13, 7, 8, 6, 5};
    sortByFrequency(a);
}