*Top K Frequent Elements

public class Solution {
public List<Integer> topKFrequent(int[] nums, int k) {
    //count the frequency for each element
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    for(int num: nums){
        if(map.containsKey(num)){
            map.put(num, map.get(num)+1);
        }else{
            map.put(num, 1);
        }
    }
 
    //get the max frequency
    int max = 0;
    for(Map.Entry<Integer, Integer> entry: map.entrySet()){
        max = Math.max(max, entry.getValue());
    }
 
    //initialize an array of ArrayList. index is frequency, value is list of numbers
    ArrayList<Integer>[] arr = (ArrayList<Integer>[]) new ArrayList[max+1];
    for(int i=1; i<=max; i++){
        arr[i]=new ArrayList<Integer>();
    }
 
    for(Map.Entry<Integer, Integer> entry: map.entrySet()){
        int count = entry.getValue();
        int number = entry.getKey();
        arr[count].add(number);
    }
 
    List<Integer> result = new ArrayList<Integer>();
 
    //add most frequent numbers to result
    for(int j=max; j>=1; j--){
        if(arr[j].size()>0){
            for(int a: arr[j]){
                result.add(a);
            }
        }
 
        if(result.size()==k)
            break;
    }
 
    return result;
}
}