public static List<int> find_top_k_frequent_elements(List<int> arr, int k) {
        List<int> Output = new List<int>();

        Dictionary<int,int> numsList = new Dictionary<int,int>();

        foreach(int ele in arr){
            if(numsList.ContainsKey(ele))
                numsList[ele] += 1;
            else
                numsList.Add(ele,1);
        }

        Dictionary<int,List<int>> dictFreq = new Dictionary<int,List<int>>();
        int highestFreq = 0;
        foreach(KeyValuePair<int, int> kvp in numsList){

            highestFreq = Math.Max(highestFreq, kvp.Value);
            if(dictFreq.ContainsKey(kvp.Value)){
                List<int> numbers = dictFreq[kvp.Value];
                numbers.Add(kvp.Key);
            }
            else{
                List<int> numbers = new List<int>();
                numbers.Add(kvp.Key);
                dictFreq.Add(kvp.Value, numbers);
            }

        }

        for(int freq=highestFreq; freq >=1; freq--)
        {
            if(dictFreq.ContainsKey(freq))
            {
                foreach(var n in dictFreq[freq])
                {
                    Output.Add(n);
                    if(Output.Count == k)
                        return Output;
                }
            }
        }

        return Output;
    }