Sort By Frequency - Hashing Problem

Problem : Sort elements by their frequency.

Following is the implementation where we store the frequency in hashmap:

	package hashTables.problems;
	import java.util.ArrayList;
	import java.util.LinkedHashMap;
	import java.util.Map;
	public class SortByFrequency {
	public static ArrayList<Integer> sortByFrequency(int arr[]) {
	int l1 = arr.length;
	Map<Integer, Integer> countMap = new LinkedHashMap<>();
	for (int i = 0; i < l1; i++) {
	if (countMap.containsKey(arr[i])) {
	countMap.put(arr[i], countMap.get(arr[i]) + 1);
	} else {
	countMap.put(arr[i], 1);
	}
	}
	ArrayList<Integer> result = new ArrayList<Integer>();
	countMap.entrySet().stream().sorted(Map.Entry.<Integer, Integer>comparingByValue().reversed().thenComparing(Map.Entry.<Integer, Integer>comparingByKey())).forEach(e -> {
	int key = e.getKey();
	int val = e.getValue();
	for (int i = 0; i < val; i++) {
	result.add(key);
	}
	});
	return result;
	}
	public static void main(String[] args) {
	int arr1[]= {6,6,2,3,1,4,1,5,6,2,8,7,4,2,1,3,4,5,9,6};
	ArrayList<Integer> result = sortByFrequency(arr1);
	for(int i: result) {
	System.out.println(i);
	}
	}
	}

view raw SortByFrequency.java hosted with ❤ by GitHub