What am I doing wrong in this C# mergesort algorithm?

Question

For a school assignment, I have to implement mergesort.

I've used this code to do the trick:

static int[] MergeSort(int[] C)
    {
        int left = 0;
        int right = C.Length;
        int middle = (left + right) / 2;
        int[] A, B;
        A = new int[middle];
        B = new int[middle];

        if (C.Length == 0 || C.Length == 1)
        {
            return C;
        }
        else
        {
            for (int i = left; i < middle; i++)
            {
                A[i] = C[i];
                B[i] = C[middle + i];
            }

            MergeSort(A);
            MergeSort(B);
            return Merge(A, B, C);
        }
    }

    static int[] Merge(int[] A, int[] B, int[] C)
    {
        int i, j, k;
        i = j = k = 0;
        int n = A.Length;
        int m = B.Length;
        int c = C.Length;
        int middle = C.Length / 2;

        while (i < n && j < m)
        {
            if (A[i] < B[j])
            {
                C[k] = A[i];
                i++;
            }
            else
            {
                C[k] = B[j];
                j++;
            }
            k++;

            if (i == n)
            {
                for (int b = i; b < B.Length; b++)
                {
                    C[middle + b] = B[b];
                }
            }
            else
            {
                for (int a = i; a < A.Length; a++)
                {
                    C[middle + a] = A[a];
                }
            }
        }

        return C;
    }

It does not work for a lot of different rows though. I've already debugged and checked if there was something wrong with constraints, but I can't seem to find the problem.

Thanks in advance!

Answer 1

In addition to Guffa answer, you should edit the code of the Merge method like this:

    while (i < n && j < m)
    {
        if (A[i] < B[j])
        {
            C[k] = A[i];
            i++;
        }
        else
        {
            C[k] = B[j];
            j++;
        }
        k++;
    }
    if (i == n)
    {
        for (int b = j; b < B.Length; b++)
        {
            C[k++] = B[b];
        }
    }
    else
    {
        for (int a = i; a < A.Length; a++)
        {
            C[k++] = A[a];
        }
    }

While loop block should end right after k++ and in first for loop you should initialize b with j instead of i. Also, watch out for the index of the next element of the C array, it is k, not necessarily middle + a or b.

Answer 2

The first thing I spot is how you split the array into two:

A = new int[middle];
B = new int[middle];

If the length is not even, you will be leaving out the last item. You should have:

A = new int[middle];
B = new int[right - middle];

Then you would use separate loops for them, as they can be different in length:

for (int i = left; i < middle; i++) {
  A[i - left] = C[i];
}
for (int i = middle; i < right; i++) {
  B[i - middle] = C[i];
}