c# merge sort algorithm implementation

        private int[] MergeSort(int[] a)
        {
            if ( a.Length == 1)
                return a;
            int middle = a.Length / 2;
            int[] left = new int[middle];
            for (int i = 0 ; i < middle ; i ++)
            {
                left[ i ] = a[ i ];
            }
            int[] right = new int[a.Length - middle];
            for( int i = 0; i < a.Length - middle; i++ )
            {
                right[i] = a[i+middle];
            }
            left = MergeSort( left );
            right = MergeSort( right );

            int leftptr = 0;
            int rightptr = 0;

            int[] sorted = new int[a.Length];
            for(int k = 0 ; k < a.Length; k++)
            {
                if ( rightptr == right.Length || ((leftptr < left.Length ) && (left[leftptr] <= right[rightptr])))
                {
                    sorted[ k ] = left[ leftptr ];
                    leftptr++;
                }
                else if( leftptr == left.Length || ((rightptr < right.Length ) && (right[rightptr] <= left[leftptr] )))
                {
                    sorted[k] = right[rightptr];
                    rightptr++;
                }
            }
            return sorted;            
        }