Question

So I’ve been trying to implement a quicksort myself, just to learn something from it, but it also generates a stackoverflowexception, but I can’t seem to find what the cause is.

Can someone give me a clue?

  public void Partition(List<int> valuelist, out List<int> greater, out List<int> lesser)
        {
            lesser = new List<int>();  // <-- Stackoverflow exception here!
            greater = new List<int>();

            if (valuelist.Count <= 1)
                return;

            pivot = valuelist.First();

            foreach (int Element in valuelist)
            {
                if (Element <= pivot)
                    lesser.Add(Element);
                else
                    greater.Add(Element);
            }
        }

        public List<int> DoQuickSort(List<int> list)
        {
            List<int> great;
            List<int> less;

            Partition(list, out great, out less);

            DoQuickSort(great);
            DoQuickSort(less);

            list.Clear();
            list = (List<int>)less.Concat(great);

            return list;

        }

Answer 1

You’re not putting any conditions on your recursive calls to DoQuicksort, so it’ll never stop recursing, leading to a stack overflow. You should only be calling DoQuicksort on a list if it contains more than one element.

Edit: As Will said in his comment, this is a very slow approach to “Quicksort”. You should look at in-place partitioning algorithms, as mentioned on Wikipedia’s Quicksort article.