Recursive QuickSort stackoverflowexception

class Program
{
    static void Main(string[] args)
    {
        int x = 3;
        List<int> num = new List<int>();
        Random rnd = new Random();
        for (int i = 0; i < x; i++)
        {
            num.Add(rnd.Next(99));
        }
        Sort s = new Sort();
        int q = 1, w = 0;

        s.QuickSort<int>(num);


    }
}

public class Sort
{
    public Sort()
    {
    }
    public void QuickSort<T>(List<T> Tlist) where T : IComparable
    {
        List<T> tmp = QuickSortHelper(Tlist);
        Tlist.Clear();
        foreach (T t in tmp)
            Tlist.Add(t);
    }
    public List<T> QuickSortHelper<T>(List<T> Tlist)  where T : IComparable
    {
        List<T> left= new List<T>(), right = new List<T>(), returList = new List<T>();
        if (Tlist.Count <= 1)
            return Tlist;
        int x =Tlist.Count-1;
        T pivot = Tlist[x];
        foreach (T t in Tlist)
            if (t.CompareTo(pivot) == -1)
                left.Add(t);
            else right.Add(t);
        foreach(T t in QuickSortHelper<T>(left))
            returList.Add(t);
        returList.Add(pivot);
        foreach (T t in QuickSortHelper<T>(right))
            returList.Add(t);
        return returList;
    }