#VB.NET I found this when I did your basic merge sort when I came across a StackOverFlowError. I was so confuzzled I asked a SO question about it. Without going to the link, can you find the cause? The link. Obviously this sorting algorithm uses recursion, but that is not the cause of the error being thrown.

Runner Class:

Module Module1

    Sub Main()
        dim array = new integer(100){}
        dim random = new Random()
        for i=0 to 100
            array(i) = random.next()
        next
        dim sorter = new MergeSort()
        sorter.sort(array)
    End Sub
End Module

MergeSort Class:

public class MergeSort

    private numbers() as integer
    private helper() as integer

    private number as integer

    public sub sort(values as integer())
        me.numbers = values
        number = values.length
        me.helper = new integer(number) {}
        mergesort(0, number - 1)
    end sub

    private sub mergesort(low as integer, high as integer)
' check if low is smaller then high, if not then the array is sorted
        if (low < high)
  ' Get the index of the element which is in the middle
          dim middle as integer = low + (high - low) / 2
  ' Sort the left side of the array
          mergesort(low, middle)
  ' Sort the right side of the array
          mergesort(middle + 1, high)
  ' Combine them both
          merge(low, middle, high)
        end if
    end sub

    private sub merge(low as integer,middle as integer,high as integer)
    ' Copy both parts into the helper array
        for l = low to high 
          helper(l) = numbers(l)
        next

        dim i as integer = low
        dim j as integer = middle + 1
        dim k as integer = low
    ' Copy the smallest values from either the left or the right side back
    ' to the original array
        while (i <= middle andalso j <= high) 
          if (helper(i) <= helper(j)) 
            numbers(k) = helper(i)
            i+=1
          else 
            numbers(k) = helper(j)
            j+=1
          end if
          k+=1
        end while

    ' Copy the rest of the left side of the array integero the target array
        while (i <= middle) 
          numbers(k) = helper(i)
          k+=1
          i+=1
        end while
    end sub
end class