JavaAlgorithms/Maths/Mode.java

package Maths;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;

/*
 * Find the mode of an array of numbers
 * 
 * The mode of an array of numbers is the most frequently occurring number in the array, 
 * or the most frequently occurring numbers if there are multiple numbers with the same frequency
 */
public class Mode {
	
	public static void main(String[] args) {
		
		/* Test array of integers */
		assert (mode(new int[]{})) == null;
		assert Arrays.equals(mode(new int[]{5}), new int[]{5});
		assert Arrays.equals(mode(new int[]{1, 2, 3, 4, 5}), new int[]{1, 2, 3, 4, 5});
		assert Arrays.equals(mode(new int[]{7, 9, 9, 4, 5, 6, 7, 7, 8}), new int[]{7});
		assert Arrays.equals(mode(new int[]{7, 9, 9, 4, 5, 6, 7, 7, 9}), new int[]{7, 9});
		
	}
	
	/*
	 * Find the mode of an array of integers
	 * 
	 * @param numbers array of integers
	 * @return mode of the array
	 */
	public static int[] mode(int[] numbers) {
		
		if(numbers.length == 0) return null;
		
		HashMap<Integer, Integer> count = new HashMap<>();
		
		for(int num : numbers) {
			if(count.containsKey(num)) {
				
				count.put(num, count.get(num) + 1);
				
			} else {
				
				count.put(num, 1);
				
			}
			
		}
		
		int max = Collections.max(count.values());
		ArrayList<Integer> modes = new ArrayList<>();
		
		for(int num : count.keySet()) {
			if(count.get(num) == max) {
				modes.add(num);
			}
		}
		return modes.stream().mapToInt(n -> n).toArray();
	}
	
}