TopKFrequent.cs

public static int[] TopKFrequent(int[] nums, int k)
{
    if (nums.Length == 0 || k == 0)
        return Array.Empty<int>();

    var result = new List<int>();
    Dictionary<int, int> dic = new Dictionary<int, int>();
    foreach (var item in nums)
    {
        if (dic.ContainsKey(item))
            dic[item]++;
        else
            dic.Add(item, 1);
    }

    return dic.OrderByDescending(x => x.Value).Select(x => x.Key).Take(k).ToArray();
}