double stringToDouble(string num)

 {

     bool minus = false;      //标记是否是负数

     string real = num;       //real表示num的绝对值

     if (num.at() == '-')

     {

         minus = true;

         real = num.substr(, num.size()-);

     }

     char c;

     int i = ;

     double result = 0.0 , dec = 10.0;

     bool isDec = false;       //标记是否有小数

     unsigned long size = real.size();

     while(i < size)

     {

         c = real.at(i);

         if (c == '.')

         {//包含小数

             isDec = true;

             i++;

             continue;

         }

         if (!isDec)

         {

             result = result* + c - '';

         }

         else

         {//识别小数点之后都进入这个分支

             result = result + (c - '')/dec;

             dec *= ;

         }

         i++;

     }

     if (minus == true) {

         result = -result;

     }

     return result;

 }

string doubleToString(double num)

{

    char str[];

    sprintf(str, "%lf", num);

    string result = str;

    return result;

}