Get the three most frequent numbers in a list.

numbers.java

import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.Iterator;

public class numbers {
  	
	public static void main(String[] args) {
		
		class ValueComparator implements Comparator<Integer> {
		    Map<Integer, Integer> base;
		    public ValueComparator(Map<Integer, Integer> base) {
		        this.base = base;
		    }
		    // Note: this comparator imposes orderings that are inconsistent with equals.    
		    public int compare(Integer a, Integer b) {
		        if (base.get(a) >= base.get(b)) {
		            return -1;
		        } else {
		            return 1;
		        } // returning 0 would merge keys
		    }
		}
		
		List<Integer> numbers = Arrays.asList(1,2,3,4,1,6,7,3,4,7,7,8,9,1,2,1,3,3,3,3,3,3,3);
		HashMap<Integer,Integer> counts = new HashMap<Integer,Integer>();
		ValueComparator vc =  new ValueComparator(counts);
		TreeMap<Integer,Integer> sortedCounts = new TreeMap<Integer,Integer>(vc);
		
		for (Integer i: numbers) {
			if (counts.get(i) == null) {
				counts.put(i, 1);
			} else {
				counts.put(i, counts.get(i) + 1);
			}
		}
		
		sortedCounts.putAll(counts);
			
		int x = 0;
		Iterator<Map.Entry<Integer,Integer>> sortedCountsIterator = sortedCounts.entrySet().iterator();
		while( sortedCountsIterator.hasNext() && x < 3 ){
			System.out.println(sortedCountsIterator.next());
			x += 1;
		}
			
	}

}

numbers.rb

x = [1,2,3,4,1,6,7,3,4,7,7,8,9,1,2,1,3,3,3,3,3,3,3]
x.inject({}) { |counts, x| counts[x] = 0 unless counts[x]; counts[x] += 1; counts }.sort_by{ |key,val| val }.reverse.first(3)