Question: Find the highest/lowest frequency element.If there are multiple elements that have the highest frequency or lowest frequency then pick the smallest element.

Coding Ninjas Problem link 

import java.util.HashMap;

import java.util.Map;

import java.util.TreeSet;

public class Solution {

    public static int[] getFrequencies(int[] v) {

        int n = v.length;

        return frequency(v, n);

    }

    public static int[] frequency(int[] arr, int n) {

        HashMap<Integer, Integer> map = new HashMap<>();

        for (int i = 0; i < n; i++) {

            map.put(arr[i], map.getOrDefault(arr[i], 0) + 1);

        }

        TreeSet<Integer> maxFreqElements = new TreeSet<>();

        TreeSet<Integer> minFreqElements = new TreeSet<>();

        int maxFreq = 0, minFreq = n;

        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {

            int count = entry.getValue();

            int element = entry.getKey();

            if (count > maxFreq) {

                maxFreqElements.clear();

                maxFreqElements.add(element);

                maxFreq = count;

            } else if (count == maxFreq) {

                maxFreqElements.add(element);

            }

            if (count < minFreq) {

                minFreqElements.clear();

                minFreqElements.add(element);

                minFreq = count;

            } else if (count == minFreq) {

                minFreqElements.add(element);

            }

        }

        int[] res = new int[2];

        res[1] = minFreqElements.first(); // Smallest element with minimum frequency

        if (maxFreqElements.size() > 1) {

            res[0] = maxFreqElements.first(); // Smallest element with maximum frequency

        } else {

            res[0] = maxFreqElements.pollFirst();

        }

        return res;

    }

}