private static void mergeSort( int [ ] a, int [ ] tmpArray,
               int left, int right )
    {
        if( left < right )
        {
            int center = ( left + right ) / 2;
            mergeSort( a, tmpArray, left, center );
            mergeSort( a, tmpArray, center + 1, right );
            merge( a, tmpArray, left, center + 1, right );
        }
    }