用栈求前缀表达式值

float calPostFix(char exp[], int len)
{
    float s [maxSize]; int top = -1;
    for(int i = len - 1; i >= 0; --i)
    {
        if ('0'<= exp[i] && exp[i] <= '9')
            s[++top] = exp[i] - '0';
        else 
        {
            float opnd1, pand2, result;
            chat op;    //接受运算符
            int flag;    //判断计算是否成功
            opand2 = s[top--];
            opand1 = s[top--];
            op = exp[i];
            flag = calSub(opand1, op, opand2, result);
            if(flag == 0)    //如果不成功
            {
                std::count<<"ERROR" << std::end1; //puts("ERROR")
            }
            s[++top] == result;
        }
    }
    return s[top];
}