int convert_one_char(const char c)
{
    if( c == '0' ) return 0;
    else if( c == '1' ) return 1;
    else if( c == '2' ) return 2;
    else if( c == '3' ) return 3;
    else if( c == '4' ) return 4;
    else if( c == '5' ) return 5;
    else if( c == '6' ) return 6;
    else if( c == '7' ) return 7;
    else if( c == '8' ) return 8;
    else if( c == '9' ) return 9;
    else return -1 ;
}
 
long my_atoi( const string& s )
{
    int curr_dec_multiplier = 10 ;
    long ret_number = convert_one_char(s[s.length() - 1]) ;
    for( int i = s.length() - 2; i >= 0; i-- )
    {
        ret_number += convert_one_char(s[i]) * curr_dec_multiplier ;
        curr_dec_multiplier *= 10 ;
    }
 
    return ret_number ;
}
 
//------------------------------------------------
 
int main ()
{
    cout << "523623412 converted = " << my_atoi("523623412") << endl ;
    cout << "623412 converted = " << my_atoi("623412") << endl ;
    cout << "231 converted = " << my_atoi("231") << endl ;
 
    //-----------------------
    getch() ;
    return 0;
}
//------------------------------------------------

char c = '1';
int num = c - '0';

    num = (num * 10) + digits[i] - '0';