get top k frequent elements ( Priority Queue implementation ), Java

Language: Layout:

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    public List<Integer> topKFrequent(int[] nums, int k) {
        Map<Integer,Integer> mp = new HashMap<>();
        for(int i:nums){
            mp.put(i,mp.containsKey(i)?mp.get(i)+1:1);
        }
        PriorityQueue<Tuple> pq = new PriorityQueue<Tuple>(mp.size()+2,new CustomComparer());
        for(int key : mp.keySet()){
            pq.add(new Tuple(key,mp.get(key)));
        }
        List<Integer> out = new ArrayList<>();
        for(int i=0;i<k;i++){
            out.add(pq.poll().item);
        }
        return out;
    }
}
class Tuple{
    int item;
    int count;
    Tuple(int item,int count){
        this.item = item;
        this.count = count;
    }
}
class CustomComparer implements Comparator<Tuple>{
    public int compare(Tuple t1, Tuple t2) { 
           if (t1.count < t2.count) 
               return 1; 
           else if (t1.count > t2.count) 
               return -1; 
           return 0; 
    } 
}

[ + ] Show input