https://www.nowcoder.com/practice/1277c681251b4372bdef344468e4f26e?tpId=13&&tqId=11202&rp=6&ru=/activity/oj&qru=/ta/coding-interviews/question-ranking

class Solution {
public:
    int StrToInt(string str) {
        if(str.empty())
        {
            return 0;
        }
        //  遍历一遍，判断字符串是否符合
        for(size_t i=1; i<str.size(); ++i)
        {
            if(!('0'<=str[i] && str[i]<='9'))
            {
                return 0;
            }
        }

        //  字符串符合
        string s;
        int flag=0;
        if('0'<=str[0] && str[0]<='9')
        {
            s=str;
        }
        else if(str[0]=='-')
        {
            flag=1;
            s=str.substr(1);
        }
        else if(str[0]=='+')
        {
            s=str.substr(1);
        }
        else
        {
            return 0;
        }

        int num = PrintNum(s);    //  数字字符转整数
        if(flag)
        {
            num=-num;
        }

        return num;
    }
    //  数字字符转整数
    int PrintNum(string& str)
    {
        int count=0;
        for(auto ch:str)
        {
            count=(count*10)+(ch-'0');
        }

        return count;
    }
};