大数相乘ADT（String）

string BigNumMultipy( string str1, string str2 )  
{  
    int len1 = str1.length();  
    int len2 = str2.length();  
    vector< int > res( len1 + len2, 0 );  
    reverse( str1.begin(), str1.end() );  
    reverse( str2.begin(), str2.end() );  
    int i, j, k;  
    for( i = 0; i < len1; i++ )  
    {  
        for( j = 0; j < len2; j++ )  
        {  
            res[i+j] += ( str1[i] - '0' ) * ( str2[j] - '0' );  
        }  
    }  
    for( k = 0; k < len1 + len2; k++ )  
    {  
        if( res[k] >= 10 )  
        {  
            res[k+1] += res[k] / 10;  //这两句顺序一定要正确，先算进位，然后才是本位。  
            res[k] %= 10;  
        }  
    }  
    string resStr = "";  
    for( k = 0; k < len1 + len2; k++ )  
    {  
        char c = '0' + res[k];  
        resStr += c;  
    }  
  
    reverse( resStr.begin(), resStr.end() );  
    return resStr;  
}