void multiply( int multiplicand[], int multiplier[], int product[],int multiplicandSize, int multiplierSize, int &productSize )
{
    memset(product, 0, arraySize*sizeof(int));
    for( int i = 0; i < multiplicandSize; i++ ) // 先乘
        for( int j = 0; j < multiplierSize; j++ )
            product[i+j] += multiplicand[i] * multiplier[j];

    for( int i = 0; i < multiplicandSize + multiplierSize; i++ ) // 進位
    {
        if( product[i] > 9 )
        {
            product[i+1] += product[i] / 10;
            product[i] %= 10;
        }
    }

    productSize = multiplicandSize + multiplierSize; // 算size
    if( product[productSize-1] == 0 )
        productSize -= 1;
}