程序代码：

#include <stdio.h>

static void foo_( unsigned a, unsigned n, unsigned carry )
{
    if( n < 2  )
    {
        putchar( '0' + n*a - carry );
        return;
    }
    foo_( a, n-1, ((n-2)*a+carry + 9)/10 );
    putchar( '0' + (10 - ((n-2)*a+carry)%10)%10 );
}
void foo( unsigned a, unsigned n )
{
    foo_( a, n, 0 );
    putchar( '\n' );
}

int main( void )
{
    foo( 2, 3 ); // 198
    foo( 2, 2 ); // 20
    foo( 2, 1 ); // 2
    foo( 2, 0 ); // 0

    foo( 8, 99 ); // 790123456790123456790123456790123456790123456790123456790123456790123456790123456790123456790123544
}