Multiply the numbers . It multiplies each digit of second string to each digit of first and stores the result .
for ( int i = n - 1 ; i >= 0 ; i -- ) for ( int j = m - 1 ; j >= 0 ; j -- ) result [ i + j + 1 ] += ( s1 [ i ] - '0' ) * ( s2 [ j ] - '0' ) ;