17.把字符串转换成整数atoi

bool bValid = true;

int str2int_core(const char *digit,bool minus)
{
    long long sum = 0;
    int flag = minus ? -1:1;
    while(*digit!=‘\0‘)
    {
        if (*digit>=‘0‘&&*digit<=‘9‘)
        {
            sum = 10*sum + flag*(*digit-‘0‘);

            // whether overflow ?
            if ((!minus && sum > (signed int)0x7fffffff)
                || (minus && sum < (signed int)0x80000000))
            {
                sum = 0;
                break;
            }
            digit ++;
        }
        else
        {
            sum = 0;
            break;
        }
    }

    if (*digit==‘\0‘)
    {
        bValid = true;
    }
    return (int)sum;
}

int str2int(const char *str)
{
    bValid = false;
    long long sum = 0;
    if (NULL==str || *str==‘\0‘)
        return (int)sum;
    bool minus = false;
    if (*str==‘+‘)
    {
        str++;
    }
    else if (*str==‘-‘)
    {
        str++;
        minus = true;
    }
    if (*str==‘\0‘)
        return  (int)sum;

    return str2int_core(str,minus);
}

void test_case(const char *str)
{
    int sum = str2int(str);
    if (bValid)
        cout<<sum<<endl;
    else
        cout<<"Invalid input!"<<endl;
}